Adding upstream version 5.10.209.upstream/5.10.209

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 3040 insertions, 0 deletions

diff --git a/mm/gup.c b/mm/gup.c
new file mode 100644
index 000000000..11307a8b2
--- /dev/null
+++ b/mm/gup.c
@@ -0,0 +1,3040 @@
+// SPDX-License-Identifier: GPL-2.0-only
+#include <linux/kernel.h>
+#include <linux/errno.h>
+#include <linux/err.h>
+#include <linux/spinlock.h>
+
+#include <linux/mm.h>
+#include <linux/memremap.h>
+#include <linux/pagemap.h>
+#include <linux/rmap.h>
+#include <linux/swap.h>
+#include <linux/swapops.h>
+
+#include <linux/sched/signal.h>
+#include <linux/rwsem.h>
+#include <linux/hugetlb.h>
+#include <linux/migrate.h>
+#include <linux/mm_inline.h>
+#include <linux/sched/mm.h>
+
+#include <asm/mmu_context.h>
+#include <asm/tlbflush.h>
+
+#include "internal.h"
+
+struct follow_page_context {
+	struct dev_pagemap *pgmap;
+	unsigned int page_mask;
+};
+
+static void hpage_pincount_add(struct page *page, int refs)
+{
+	VM_BUG_ON_PAGE(!hpage_pincount_available(page), page);
+	VM_BUG_ON_PAGE(page != compound_head(page), page);
+
+	atomic_add(refs, compound_pincount_ptr(page));
+}
+
+static void hpage_pincount_sub(struct page *page, int refs)
+{
+	VM_BUG_ON_PAGE(!hpage_pincount_available(page), page);
+	VM_BUG_ON_PAGE(page != compound_head(page), page);
+
+	atomic_sub(refs, compound_pincount_ptr(page));
+}
+
+/* Equivalent to calling put_page() @refs times. */
+static void put_page_refs(struct page *page, int refs)
+{
+#ifdef CONFIG_DEBUG_VM
+	if (VM_WARN_ON_ONCE_PAGE(page_ref_count(page) < refs, page))
+		return;
+#endif
+
+	/*
+	 * Calling put_page() for each ref is unnecessarily slow. Only the last
+	 * ref needs a put_page().
+	 */
+	if (refs > 1)
+		page_ref_sub(page, refs - 1);
+	put_page(page);
+}
+
+/*
+ * Return the compound head page with ref appropriately incremented,
+ * or NULL if that failed.
+ */
+static inline struct page *try_get_compound_head(struct page *page, int refs)
+{
+	struct page *head = compound_head(page);
+
+	if (WARN_ON_ONCE(page_ref_count(head) < 0))
+		return NULL;
+	if (unlikely(!page_cache_add_speculative(head, refs)))
+		return NULL;
+
+	/*
+	 * At this point we have a stable reference to the head page; but it
+	 * could be that between the compound_head() lookup and the refcount
+	 * increment, the compound page was split, in which case we'd end up
+	 * holding a reference on a page that has nothing to do with the page
+	 * we were given anymore.
+	 * So now that the head page is stable, recheck that the pages still
+	 * belong together.
+	 */
+	if (unlikely(compound_head(page) != head)) {
+		put_page_refs(head, refs);
+		return NULL;
+	}
+
+	return head;
+}
+
+/*
+ * try_grab_compound_head() - attempt to elevate a page's refcount, by a
+ * flags-dependent amount.
+ *
+ * "grab" names in this file mean, "look at flags to decide whether to use
+ * FOLL_PIN or FOLL_GET behavior, when incrementing the page's refcount.
+ *
+ * Either FOLL_PIN or FOLL_GET (or neither) must be set, but not both at the
+ * same time. (That's true throughout the get_user_pages*() and
+ * pin_user_pages*() APIs.) Cases:
+ *
+ *    FOLL_GET: page's refcount will be incremented by 1.
+ *    FOLL_PIN: page's refcount will be incremented by GUP_PIN_COUNTING_BIAS.
+ *
+ * Return: head page (with refcount appropriately incremented) for success, or
+ * NULL upon failure. If neither FOLL_GET nor FOLL_PIN was set, that's
+ * considered failure, and furthermore, a likely bug in the caller, so a warning
+ * is also emitted.
+ */
+static __maybe_unused struct page *try_grab_compound_head(struct page *page,
+							  int refs,
+							  unsigned int flags)
+{
+	if (flags & FOLL_GET)
+		return try_get_compound_head(page, refs);
+	else if (flags & FOLL_PIN) {
+		int orig_refs = refs;
+
+		/*
+		 * Can't do FOLL_LONGTERM + FOLL_PIN with CMA in the gup fast
+		 * path, so fail and let the caller fall back to the slow path.
+		 */
+		if (unlikely(flags & FOLL_LONGTERM) &&
+				is_migrate_cma_page(page))
+			return NULL;
+
+		/*
+		 * CAUTION: Don't use compound_head() on the page before this
+		 * point, the result won't be stable.
+		 */
+		page = try_get_compound_head(page, refs);
+		if (!page)
+			return NULL;
+
+		/*
+		 * When pinning a compound page of order > 1 (which is what
+		 * hpage_pincount_available() checks for), use an exact count to
+		 * track it, via hpage_pincount_add/_sub().
+		 *
+		 * However, be sure to *also* increment the normal page refcount
+		 * field at least once, so that the page really is pinned.
+		 */
+		if (hpage_pincount_available(page))
+			hpage_pincount_add(page, refs);
+		else
+			page_ref_add(page, refs * (GUP_PIN_COUNTING_BIAS - 1));
+
+		mod_node_page_state(page_pgdat(page), NR_FOLL_PIN_ACQUIRED,
+				    orig_refs);
+
+		return page;
+	}
+
+	WARN_ON_ONCE(1);
+	return NULL;
+}
+
+static void put_compound_head(struct page *page, int refs, unsigned int flags)
+{
+	if (flags & FOLL_PIN) {
+		mod_node_page_state(page_pgdat(page), NR_FOLL_PIN_RELEASED,
+				    refs);
+
+		if (hpage_pincount_available(page))
+			hpage_pincount_sub(page, refs);
+		else
+			refs *= GUP_PIN_COUNTING_BIAS;
+	}
+
+	put_page_refs(page, refs);
+}
+
+/**
+ * try_grab_page() - elevate a page's refcount by a flag-dependent amount
+ *
+ * This might not do anything at all, depending on the flags argument.
+ *
+ * "grab" names in this file mean, "look at flags to decide whether to use
+ * FOLL_PIN or FOLL_GET behavior, when incrementing the page's refcount.
+ *
+ * @page:    pointer to page to be grabbed
+ * @flags:   gup flags: these are the FOLL_* flag values.
+ *
+ * Either FOLL_PIN or FOLL_GET (or neither) may be set, but not both at the same
+ * time. Cases:
+ *
+ *    FOLL_GET: page's refcount will be incremented by 1.
+ *    FOLL_PIN: page's refcount will be incremented by GUP_PIN_COUNTING_BIAS.
+ *
+ * Return: true for success, or if no action was required (if neither FOLL_PIN
+ * nor FOLL_GET was set, nothing is done). False for failure: FOLL_GET or
+ * FOLL_PIN was set, but the page could not be grabbed.
+ */
+bool __must_check try_grab_page(struct page *page, unsigned int flags)
+{
+	WARN_ON_ONCE((flags & (FOLL_GET | FOLL_PIN)) == (FOLL_GET | FOLL_PIN));
+
+	if (flags & FOLL_GET)
+		return try_get_page(page);
+	else if (flags & FOLL_PIN) {
+		int refs = 1;
+
+		page = compound_head(page);
+
+		if (WARN_ON_ONCE(page_ref_count(page) <= 0))
+			return false;
+
+		if (hpage_pincount_available(page))
+			hpage_pincount_add(page, 1);
+		else
+			refs = GUP_PIN_COUNTING_BIAS;
+
+		/*
+		 * Similar to try_grab_compound_head(): even if using the
+		 * hpage_pincount_add/_sub() routines, be sure to
+		 * *also* increment the normal page refcount field at least
+		 * once, so that the page really is pinned.
+		 */
+		page_ref_add(page, refs);
+
+		mod_node_page_state(page_pgdat(page), NR_FOLL_PIN_ACQUIRED, 1);
+	}
+
+	return true;
+}
+
+/**
+ * unpin_user_page() - release a dma-pinned page
+ * @page:            pointer to page to be released
+ *
+ * Pages that were pinned via pin_user_pages*() must be released via either
+ * unpin_user_page(), or one of the unpin_user_pages*() routines. This is so
+ * that such pages can be separately tracked and uniquely handled. In
+ * particular, interactions with RDMA and filesystems need special handling.
+ */
+void unpin_user_page(struct page *page)
+{
+	put_compound_head(compound_head(page), 1, FOLL_PIN);
+}
+EXPORT_SYMBOL(unpin_user_page);
+
+/**
+ * unpin_user_pages_dirty_lock() - release and optionally dirty gup-pinned pages
+ * @pages:  array of pages to be maybe marked dirty, and definitely released.
+ * @npages: number of pages in the @pages array.
+ * @make_dirty: whether to mark the pages dirty
+ *
+ * "gup-pinned page" refers to a page that has had one of the get_user_pages()
+ * variants called on that page.
+ *
+ * For each page in the @pages array, make that page (or its head page, if a
+ * compound page) dirty, if @make_dirty is true, and if the page was previously
+ * listed as clean. In any case, releases all pages using unpin_user_page(),
+ * possibly via unpin_user_pages(), for the non-dirty case.
+ *
+ * Please see the unpin_user_page() documentation for details.
+ *
+ * set_page_dirty_lock() is used internally. If instead, set_page_dirty() is
+ * required, then the caller should a) verify that this is really correct,
+ * because _lock() is usually required, and b) hand code it:
+ * set_page_dirty_lock(), unpin_user_page().
+ *
+ */
+void unpin_user_pages_dirty_lock(struct page **pages, unsigned long npages,
+				 bool make_dirty)
+{
+	unsigned long index;
+
+	/*
+	 * TODO: this can be optimized for huge pages: if a series of pages is
+	 * physically contiguous and part of the same compound page, then a
+	 * single operation to the head page should suffice.
+	 */
+
+	if (!make_dirty) {
+		unpin_user_pages(pages, npages);
+		return;
+	}
+
+	for (index = 0; index < npages; index++) {
+		struct page *page = compound_head(pages[index]);
+		/*
+		 * Checking PageDirty at this point may race with
+		 * clear_page_dirty_for_io(), but that's OK. Two key
+		 * cases:
+		 *
+		 * 1) This code sees the page as already dirty, so it
+		 * skips the call to set_page_dirty(). That could happen
+		 * because clear_page_dirty_for_io() called
+		 * page_mkclean(), followed by set_page_dirty().
+		 * However, now the page is going to get written back,
+		 * which meets the original intention of setting it
+		 * dirty, so all is well: clear_page_dirty_for_io() goes
+		 * on to call TestClearPageDirty(), and write the page
+		 * back.
+		 *
+		 * 2) This code sees the page as clean, so it calls
+		 * set_page_dirty(). The page stays dirty, despite being
+		 * written back, so it gets written back again in the
+		 * next writeback cycle. This is harmless.
+		 */
+		if (!PageDirty(page))
+			set_page_dirty_lock(page);
+		unpin_user_page(page);
+	}
+}
+EXPORT_SYMBOL(unpin_user_pages_dirty_lock);
+
+/**
+ * unpin_user_pages() - release an array of gup-pinned pages.
+ * @pages:  array of pages to be marked dirty and released.
+ * @npages: number of pages in the @pages array.
+ *
+ * For each page in the @pages array, release the page using unpin_user_page().
+ *
+ * Please see the unpin_user_page() documentation for details.
+ */
+void unpin_user_pages(struct page **pages, unsigned long npages)
+{
+	unsigned long index;
+
+	/*
+	 * If this WARN_ON() fires, then the system *might* be leaking pages (by
+	 * leaving them pinned), but probably not. More likely, gup/pup returned
+	 * a hard -ERRNO error to the caller, who erroneously passed it here.
+	 */
+	if (WARN_ON(IS_ERR_VALUE(npages)))
+		return;
+	/*
+	 * TODO: this can be optimized for huge pages: if a series of pages is
+	 * physically contiguous and part of the same compound page, then a
+	 * single operation to the head page should suffice.
+	 */
+	for (index = 0; index < npages; index++)
+		unpin_user_page(pages[index]);
+}
+EXPORT_SYMBOL(unpin_user_pages);
+
+#ifdef CONFIG_MMU
+static struct page *no_page_table(struct vm_area_struct *vma,
+		unsigned int flags)
+{
+	/*
+	 * When core dumping an enormous anonymous area that nobody
+	 * has touched so far, we don't want to allocate unnecessary pages or
+	 * page tables.  Return error instead of NULL to skip handle_mm_fault,
+	 * then get_dump_page() will return NULL to leave a hole in the dump.
+	 * But we can only make this optimization where a hole would surely
+	 * be zero-filled if handle_mm_fault() actually did handle it.
+	 */
+	if ((flags & FOLL_DUMP) &&
+			(vma_is_anonymous(vma) || !vma->vm_ops->fault))
+		return ERR_PTR(-EFAULT);
+	return NULL;
+}
+
+static int follow_pfn_pte(struct vm_area_struct *vma, unsigned long address,
+		pte_t *pte, unsigned int flags)
+{
+	/* No page to get reference */
+	if (flags & FOLL_GET)
+		return -EFAULT;
+
+	if (flags & FOLL_TOUCH) {
+		pte_t entry = *pte;
+
+		if (flags & FOLL_WRITE)
+			entry = pte_mkdirty(entry);
+		entry = pte_mkyoung(entry);
+
+		if (!pte_same(*pte, entry)) {
+			set_pte_at(vma->vm_mm, address, pte, entry);
+			update_mmu_cache(vma, address, pte);
+		}
+	}
+
+	/* Proper page table entry exists, but no corresponding struct page */
+	return -EEXIST;
+}
+
+/*
+ * FOLL_FORCE can write to even unwritable pte's, but only
+ * after we've gone through a COW cycle and they are dirty.
+ */
+static inline bool can_follow_write_pte(pte_t pte, unsigned int flags)
+{
+	return pte_write(pte) ||
+		((flags & FOLL_FORCE) && (flags & FOLL_COW) && pte_dirty(pte));
+}
+
+static struct page *follow_page_pte(struct vm_area_struct *vma,
+		unsigned long address, pmd_t *pmd, unsigned int flags,
+		struct dev_pagemap **pgmap)
+{
+	struct mm_struct *mm = vma->vm_mm;
+	struct page *page;
+	spinlock_t *ptl;
+	pte_t *ptep, pte;
+	int ret;
+
+	/* FOLL_GET and FOLL_PIN are mutually exclusive. */
+	if (WARN_ON_ONCE((flags & (FOLL_PIN | FOLL_GET)) ==
+			 (FOLL_PIN | FOLL_GET)))
+		return ERR_PTR(-EINVAL);
+
+	/*
+	 * Considering PTE level hugetlb, like continuous-PTE hugetlb on
+	 * ARM64 architecture.
+	 */
+	if (is_vm_hugetlb_page(vma)) {
+		page = follow_huge_pmd_pte(vma, address, flags);
+		if (page)
+			return page;
+		return no_page_table(vma, flags);
+	}
+
+retry:
+	if (unlikely(pmd_bad(*pmd)))
+		return no_page_table(vma, flags);
+
+	ptep = pte_offset_map_lock(mm, pmd, address, &ptl);
+	pte = *ptep;
+	if (!pte_present(pte)) {
+		swp_entry_t entry;
+		/*
+		 * KSM's break_ksm() relies upon recognizing a ksm page
+		 * even while it is being migrated, so for that case we
+		 * need migration_entry_wait().
+		 */
+		if (likely(!(flags & FOLL_MIGRATION)))
+			goto no_page;
+		if (pte_none(pte))
+			goto no_page;
+		entry = pte_to_swp_entry(pte);
+		if (!is_migration_entry(entry))
+			goto no_page;
+		pte_unmap_unlock(ptep, ptl);
+		migration_entry_wait(mm, pmd, address);
+		goto retry;
+	}
+	if ((flags & FOLL_NUMA) && pte_protnone(pte))
+		goto no_page;
+	if ((flags & FOLL_WRITE) && !can_follow_write_pte(pte, flags)) {
+		pte_unmap_unlock(ptep, ptl);
+		return NULL;
+	}
+
+	page = vm_normal_page(vma, address, pte);
+	if (!page && pte_devmap(pte) && (flags & (FOLL_GET | FOLL_PIN))) {
+		/*
+		 * Only return device mapping pages in the FOLL_GET or FOLL_PIN
+		 * case since they are only valid while holding the pgmap
+		 * reference.
+		 */
+		*pgmap = get_dev_pagemap(pte_pfn(pte), *pgmap);
+		if (*pgmap)
+			page = pte_page(pte);
+		else
+			goto no_page;
+	} else if (unlikely(!page)) {
+		if (flags & FOLL_DUMP) {
+			/* Avoid special (like zero) pages in core dumps */
+			page = ERR_PTR(-EFAULT);
+			goto out;
+		}
+
+		if (is_zero_pfn(pte_pfn(pte))) {
+			page = pte_page(pte);
+		} else {
+			ret = follow_pfn_pte(vma, address, ptep, flags);
+			page = ERR_PTR(ret);
+			goto out;
+		}
+	}
+
+	if (flags & FOLL_SPLIT && PageTransCompound(page)) {
+		get_page(page);
+		pte_unmap_unlock(ptep, ptl);
+		lock_page(page);
+		ret = split_huge_page(page);
+		unlock_page(page);
+		put_page(page);
+		if (ret)
+			return ERR_PTR(ret);
+		goto retry;
+	}
+
+	/* try_grab_page() does nothing unless FOLL_GET or FOLL_PIN is set. */
+	if (unlikely(!try_grab_page(page, flags))) {
+		page = ERR_PTR(-ENOMEM);
+		goto out;
+	}
+	/*
+	 * We need to make the page accessible if and only if we are going
+	 * to access its content (the FOLL_PIN case).  Please see
+	 * Documentation/core-api/pin_user_pages.rst for details.
+	 */
+	if (flags & FOLL_PIN) {
+		ret = arch_make_page_accessible(page);
+		if (ret) {
+			unpin_user_page(page);
+			page = ERR_PTR(ret);
+			goto out;
+		}
+	}
+	if (flags & FOLL_TOUCH) {
+		if ((flags & FOLL_WRITE) &&
+		    !pte_dirty(pte) && !PageDirty(page))
+			set_page_dirty(page);
+		/*
+		 * pte_mkyoung() would be more correct here, but atomic care
+		 * is needed to avoid losing the dirty bit: it is easier to use
+		 * mark_page_accessed().
+		 */
+		mark_page_accessed(page);
+	}
+	if ((flags & FOLL_MLOCK) && (vma->vm_flags & VM_LOCKED)) {
+		/* Do not mlock pte-mapped THP */
+		if (PageTransCompound(page))
+			goto out;
+
+		/*
+		 * The preliminary mapping check is mainly to avoid the
+		 * pointless overhead of lock_page on the ZERO_PAGE
+		 * which might bounce very badly if there is contention.
+		 *
+		 * If the page is already locked, we don't need to
+		 * handle it now - vmscan will handle it later if and
+		 * when it attempts to reclaim the page.
+		 */
+		if (page->mapping && trylock_page(page)) {
+			lru_add_drain();  /* push cached pages to LRU */
+			/*
+			 * Because we lock page here, and migration is
+			 * blocked by the pte's page reference, and we
+			 * know the page is still mapped, we don't even
+			 * need to check for file-cache page truncation.
+			 */
+			mlock_vma_page(page);
+			unlock_page(page);
+		}
+	}
+out:
+	pte_unmap_unlock(ptep, ptl);
+	return page;
+no_page:
+	pte_unmap_unlock(ptep, ptl);
+	if (!pte_none(pte))
+		return NULL;
+	return no_page_table(vma, flags);
+}
+
+static struct page *follow_pmd_mask(struct vm_area_struct *vma,
+				    unsigned long address, pud_t *pudp,
+				    unsigned int flags,
+				    struct follow_page_context *ctx)
+{
+	pmd_t *pmd, pmdval;
+	spinlock_t *ptl;
+	struct page *page;
+	struct mm_struct *mm = vma->vm_mm;
+
+	pmd = pmd_offset(pudp, address);
+	/*
+	 * The READ_ONCE() will stabilize the pmdval in a register or
+	 * on the stack so that it will stop changing under the code.
+	 */
+	pmdval = READ_ONCE(*pmd);
+	if (pmd_none(pmdval))
+		return no_page_table(vma, flags);
+	if (pmd_huge(pmdval) && is_vm_hugetlb_page(vma)) {
+		page = follow_huge_pmd_pte(vma, address, flags);
+		if (page)
+			return page;
+		return no_page_table(vma, flags);
+	}
+	if (is_hugepd(__hugepd(pmd_val(pmdval)))) {
+		page = follow_huge_pd(vma, address,
+				      __hugepd(pmd_val(pmdval)), flags,
+				      PMD_SHIFT);
+		if (page)
+			return page;
+		return no_page_table(vma, flags);
+	}
+retry:
+	if (!pmd_present(pmdval)) {
+		if (likely(!(flags & FOLL_MIGRATION)))
+			return no_page_table(vma, flags);
+		VM_BUG_ON(thp_migration_supported() &&
+				  !is_pmd_migration_entry(pmdval));
+		if (is_pmd_migration_entry(pmdval))
+			pmd_migration_entry_wait(mm, pmd);
+		pmdval = READ_ONCE(*pmd);
+		/*
+		 * MADV_DONTNEED may convert the pmd to null because
+		 * mmap_lock is held in read mode
+		 */
+		if (pmd_none(pmdval))
+			return no_page_table(vma, flags);
+		goto retry;
+	}
+	if (pmd_devmap(pmdval)) {
+		ptl = pmd_lock(mm, pmd);
+		page = follow_devmap_pmd(vma, address, pmd, flags, &ctx->pgmap);
+		spin_unlock(ptl);
+		if (page)
+			return page;
+	}
+	if (likely(!pmd_trans_huge(pmdval)))
+		return follow_page_pte(vma, address, pmd, flags, &ctx->pgmap);
+
+	if ((flags & FOLL_NUMA) && pmd_protnone(pmdval))
+		return no_page_table(vma, flags);
+
+retry_locked:
+	ptl = pmd_lock(mm, pmd);
+	if (unlikely(pmd_none(*pmd))) {
+		spin_unlock(ptl);
+		return no_page_table(vma, flags);
+	}
+	if (unlikely(!pmd_present(*pmd))) {
+		spin_unlock(ptl);
+		if (likely(!(flags & FOLL_MIGRATION)))
+			return no_page_table(vma, flags);
+		pmd_migration_entry_wait(mm, pmd);
+		goto retry_locked;
+	}
+	if (unlikely(!pmd_trans_huge(*pmd))) {
+		spin_unlock(ptl);
+		return follow_page_pte(vma, address, pmd, flags, &ctx->pgmap);
+	}
+	if (flags & (FOLL_SPLIT | FOLL_SPLIT_PMD)) {
+		int ret;
+		page = pmd_page(*pmd);
+		if (is_huge_zero_page(page)) {
+			spin_unlock(ptl);
+			ret = 0;
+			split_huge_pmd(vma, pmd, address);
+			if (pmd_trans_unstable(pmd))
+				ret = -EBUSY;
+		} else if (flags & FOLL_SPLIT) {
+			if (unlikely(!try_get_page(page))) {
+				spin_unlock(ptl);
+				return ERR_PTR(-ENOMEM);
+			}
+			spin_unlock(ptl);
+			lock_page(page);
+			ret = split_huge_page(page);
+			unlock_page(page);
+			put_page(page);
+			if (pmd_none(*pmd))
+				return no_page_table(vma, flags);
+		} else {  /* flags & FOLL_SPLIT_PMD */
+			spin_unlock(ptl);
+			split_huge_pmd(vma, pmd, address);
+			ret = pte_alloc(mm, pmd) ? -ENOMEM : 0;
+		}
+
+		return ret ? ERR_PTR(ret) :
+			follow_page_pte(vma, address, pmd, flags, &ctx->pgmap);
+	}
+	page = follow_trans_huge_pmd(vma, address, pmd, flags);
+	spin_unlock(ptl);
+	ctx->page_mask = HPAGE_PMD_NR - 1;
+	return page;
+}
+
+static struct page *follow_pud_mask(struct vm_area_struct *vma,
+				    unsigned long address, p4d_t *p4dp,
+				    unsigned int flags,
+				    struct follow_page_context *ctx)
+{
+	pud_t *pud;
+	spinlock_t *ptl;
+	struct page *page;
+	struct mm_struct *mm = vma->vm_mm;
+
+	pud = pud_offset(p4dp, address);
+	if (pud_none(*pud))
+		return no_page_table(vma, flags);
+	if (pud_huge(*pud) && is_vm_hugetlb_page(vma)) {
+		page = follow_huge_pud(mm, address, pud, flags);
+		if (page)
+			return page;
+		return no_page_table(vma, flags);
+	}
+	if (is_hugepd(__hugepd(pud_val(*pud)))) {
+		page = follow_huge_pd(vma, address,
+				      __hugepd(pud_val(*pud)), flags,
+				      PUD_SHIFT);
+		if (page)
+			return page;
+		return no_page_table(vma, flags);
+	}
+	if (pud_devmap(*pud)) {
+		ptl = pud_lock(mm, pud);
+		page = follow_devmap_pud(vma, address, pud, flags, &ctx->pgmap);
+		spin_unlock(ptl);
+		if (page)
+			return page;
+	}
+	if (unlikely(pud_bad(*pud)))
+		return no_page_table(vma, flags);
+
+	return follow_pmd_mask(vma, address, pud, flags, ctx);
+}
+
+static struct page *follow_p4d_mask(struct vm_area_struct *vma,
+				    unsigned long address, pgd_t *pgdp,
+				    unsigned int flags,
+				    struct follow_page_context *ctx)
+{
+	p4d_t *p4d;
+	struct page *page;
+
+	p4d = p4d_offset(pgdp, address);
+	if (p4d_none(*p4d))
+		return no_page_table(vma, flags);
+	BUILD_BUG_ON(p4d_huge(*p4d));
+	if (unlikely(p4d_bad(*p4d)))
+		return no_page_table(vma, flags);
+
+	if (is_hugepd(__hugepd(p4d_val(*p4d)))) {
+		page = follow_huge_pd(vma, address,
+				      __hugepd(p4d_val(*p4d)), flags,
+				      P4D_SHIFT);
+		if (page)
+			return page;
+		return no_page_table(vma, flags);
+	}
+	return follow_pud_mask(vma, address, p4d, flags, ctx);
+}
+
+/**
+ * follow_page_mask - look up a page descriptor from a user-virtual address
+ * @vma: vm_area_struct mapping @address
+ * @address: virtual address to look up
+ * @flags: flags modifying lookup behaviour
+ * @ctx: contains dev_pagemap for %ZONE_DEVICE memory pinning and a
+ *       pointer to output page_mask
+ *
+ * @flags can have FOLL_ flags set, defined in <linux/mm.h>
+ *
+ * When getting pages from ZONE_DEVICE memory, the @ctx->pgmap caches
+ * the device's dev_pagemap metadata to avoid repeating expensive lookups.
+ *
+ * On output, the @ctx->page_mask is set according to the size of the page.
+ *
+ * Return: the mapped (struct page *), %NULL if no mapping exists, or
+ * an error pointer if there is a mapping to something not represented
+ * by a page descriptor (see also vm_normal_page()).
+ */
+static struct page *follow_page_mask(struct vm_area_struct *vma,
+			      unsigned long address, unsigned int flags,
+			      struct follow_page_context *ctx)
+{
+	pgd_t *pgd;
+	struct page *page;
+	struct mm_struct *mm = vma->vm_mm;
+
+	ctx->page_mask = 0;
+
+	/* make this handle hugepd */
+	page = follow_huge_addr(mm, address, flags & FOLL_WRITE);
+	if (!IS_ERR(page)) {
+		WARN_ON_ONCE(flags & (FOLL_GET | FOLL_PIN));
+		return page;
+	}
+
+	pgd = pgd_offset(mm, address);
+
+	if (pgd_none(*pgd) || unlikely(pgd_bad(*pgd)))
+		return no_page_table(vma, flags);
+
+	if (pgd_huge(*pgd)) {
+		page = follow_huge_pgd(mm, address, pgd, flags);
+		if (page)
+			return page;
+		return no_page_table(vma, flags);
+	}
+	if (is_hugepd(__hugepd(pgd_val(*pgd)))) {
+		page = follow_huge_pd(vma, address,
+				      __hugepd(pgd_val(*pgd)), flags,
+				      PGDIR_SHIFT);
+		if (page)
+			return page;
+		return no_page_table(vma, flags);
+	}
+
+	return follow_p4d_mask(vma, address, pgd, flags, ctx);
+}
+
+struct page *follow_page(struct vm_area_struct *vma, unsigned long address,
+			 unsigned int foll_flags)
+{
+	struct follow_page_context ctx = { NULL };
+	struct page *page;
+
+	page = follow_page_mask(vma, address, foll_flags, &ctx);
+	if (ctx.pgmap)
+		put_dev_pagemap(ctx.pgmap);
+	return page;
+}
+
+static int get_gate_page(struct mm_struct *mm, unsigned long address,
+		unsigned int gup_flags, struct vm_area_struct **vma,
+		struct page **page)
+{
+	pgd_t *pgd;
+	p4d_t *p4d;
+	pud_t *pud;
+	pmd_t *pmd;
+	pte_t *pte;
+	int ret = -EFAULT;
+
+	/* user gate pages are read-only */
+	if (gup_flags & FOLL_WRITE)
+		return -EFAULT;
+	if (address > TASK_SIZE)
+		pgd = pgd_offset_k(address);
+	else
+		pgd = pgd_offset_gate(mm, address);
+	if (pgd_none(*pgd))
+		return -EFAULT;
+	p4d = p4d_offset(pgd, address);
+	if (p4d_none(*p4d))
+		return -EFAULT;
+	pud = pud_offset(p4d, address);
+	if (pud_none(*pud))
+		return -EFAULT;
+	pmd = pmd_offset(pud, address);
+	if (!pmd_present(*pmd))
+		return -EFAULT;
+	VM_BUG_ON(pmd_trans_huge(*pmd));
+	pte = pte_offset_map(pmd, address);
+	if (pte_none(*pte))
+		goto unmap;
+	*vma = get_gate_vma(mm);
+	if (!page)
+		goto out;
+	*page = vm_normal_page(*vma, address, *pte);
+	if (!*page) {
+		if ((gup_flags & FOLL_DUMP) || !is_zero_pfn(pte_pfn(*pte)))
+			goto unmap;
+		*page = pte_page(*pte);
+	}
+	if (unlikely(!try_grab_page(*page, gup_flags))) {
+		ret = -ENOMEM;
+		goto unmap;
+	}
+out:
+	ret = 0;
+unmap:
+	pte_unmap(pte);
+	return ret;
+}
+
+/*
+ * mmap_lock must be held on entry.  If @locked != NULL and *@flags
+ * does not include FOLL_NOWAIT, the mmap_lock may be released.  If it
+ * is, *@locked will be set to 0 and -EBUSY returned.
+ */
+static int faultin_page(struct vm_area_struct *vma,
+		unsigned long address, unsigned int *flags, int *locked)
+{
+	unsigned int fault_flags = 0;
+	vm_fault_t ret;
+
+	/* mlock all present pages, but do not fault in new pages */
+	if ((*flags & (FOLL_POPULATE | FOLL_MLOCK)) == FOLL_MLOCK)
+		return -ENOENT;
+	if (*flags & FOLL_WRITE)
+		fault_flags |= FAULT_FLAG_WRITE;
+	if (*flags & FOLL_REMOTE)
+		fault_flags |= FAULT_FLAG_REMOTE;
+	if (locked)
+		fault_flags |= FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
+	if (*flags & FOLL_NOWAIT)
+		fault_flags |= FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_RETRY_NOWAIT;
+	if (*flags & FOLL_TRIED) {
+		/*
+		 * Note: FAULT_FLAG_ALLOW_RETRY and FAULT_FLAG_TRIED
+		 * can co-exist
+		 */
+		fault_flags |= FAULT_FLAG_TRIED;
+	}
+
+	ret = handle_mm_fault(vma, address, fault_flags, NULL);
+	if (ret & VM_FAULT_ERROR) {
+		int err = vm_fault_to_errno(ret, *flags);
+
+		if (err)
+			return err;
+		BUG();
+	}
+
+	if (ret & VM_FAULT_RETRY) {
+		if (locked && !(fault_flags & FAULT_FLAG_RETRY_NOWAIT))
+			*locked = 0;
+		return -EBUSY;
+	}
+
+	/*
+	 * The VM_FAULT_WRITE bit tells us that do_wp_page has broken COW when
+	 * necessary, even if maybe_mkwrite decided not to set pte_write. We
+	 * can thus safely do subsequent page lookups as if they were reads.
+	 * But only do so when looping for pte_write is futile: in some cases
+	 * userspace may also be wanting to write to the gotten user page,
+	 * which a read fault here might prevent (a readonly page might get
+	 * reCOWed by userspace write).
+	 */
+	if ((ret & VM_FAULT_WRITE) && !(vma->vm_flags & VM_WRITE))
+		*flags |= FOLL_COW;
+	return 0;
+}
+
+static int check_vma_flags(struct vm_area_struct *vma, unsigned long gup_flags)
+{
+	vm_flags_t vm_flags = vma->vm_flags;
+	int write = (gup_flags & FOLL_WRITE);
+	int foreign = (gup_flags & FOLL_REMOTE);
+
+	if (vm_flags & (VM_IO | VM_PFNMAP))
+		return -EFAULT;
+
+	if (gup_flags & FOLL_ANON && !vma_is_anonymous(vma))
+		return -EFAULT;
+
+	if (write) {
+		if (!(vm_flags & VM_WRITE)) {
+			if (!(gup_flags & FOLL_FORCE))
+				return -EFAULT;
+			/*
+			 * We used to let the write,force case do COW in a
+			 * VM_MAYWRITE VM_SHARED !VM_WRITE vma, so ptrace could
+			 * set a breakpoint in a read-only mapping of an
+			 * executable, without corrupting the file (yet only
+			 * when that file had been opened for writing!).
+			 * Anon pages in shared mappings are surprising: now
+			 * just reject it.
+			 */
+			if (!is_cow_mapping(vm_flags))
+				return -EFAULT;
+		}
+	} else if (!(vm_flags & VM_READ)) {
+		if (!(gup_flags & FOLL_FORCE))
+			return -EFAULT;
+		/*
+		 * Is there actually any vma we can reach here which does not
+		 * have VM_MAYREAD set?
+		 */
+		if (!(vm_flags & VM_MAYREAD))
+			return -EFAULT;
+	}
+	/*
+	 * gups are always data accesses, not instruction
+	 * fetches, so execute=false here
+	 */
+	if (!arch_vma_access_permitted(vma, write, false, foreign))
+		return -EFAULT;
+	return 0;
+}
+
+/**
+ * __get_user_pages() - pin user pages in memory
+ * @mm:		mm_struct of target mm
+ * @start:	starting user address
+ * @nr_pages:	number of pages from start to pin
+ * @gup_flags:	flags modifying pin behaviour
+ * @pages:	array that receives pointers to the pages pinned.
+ *		Should be at least nr_pages long. Or NULL, if caller
+ *		only intends to ensure the pages are faulted in.
+ * @vmas:	array of pointers to vmas corresponding to each page.
+ *		Or NULL if the caller does not require them.
+ * @locked:     whether we're still with the mmap_lock held
+ *
+ * Returns either number of pages pinned (which may be less than the
+ * number requested), or an error. Details about the return value:
+ *
+ * -- If nr_pages is 0, returns 0.
+ * -- If nr_pages is >0, but no pages were pinned, returns -errno.
+ * -- If nr_pages is >0, and some pages were pinned, returns the number of
+ *    pages pinned. Again, this may be less than nr_pages.
+ * -- 0 return value is possible when the fault would need to be retried.
+ *
+ * The caller is responsible for releasing returned @pages, via put_page().
+ *
+ * @vmas are valid only as long as mmap_lock is held.
+ *
+ * Must be called with mmap_lock held.  It may be released.  See below.
+ *
+ * __get_user_pages walks a process's page tables and takes a reference to
+ * each struct page that each user address corresponds to at a given
+ * instant. That is, it takes the page that would be accessed if a user
+ * thread accesses the given user virtual address at that instant.
+ *
+ * This does not guarantee that the page exists in the user mappings when
+ * __get_user_pages returns, and there may even be a completely different
+ * page there in some cases (eg. if mmapped pagecache has been invalidated
+ * and subsequently re faulted). However it does guarantee that the page
+ * won't be freed completely. And mostly callers simply care that the page
+ * contains data that was valid *at some point in time*. Typically, an IO
+ * or similar operation cannot guarantee anything stronger anyway because
+ * locks can't be held over the syscall boundary.
+ *
+ * If @gup_flags & FOLL_WRITE == 0, the page must not be written to. If
+ * the page is written to, set_page_dirty (or set_page_dirty_lock, as
+ * appropriate) must be called after the page is finished with, and
+ * before put_page is called.
+ *
+ * If @locked != NULL, *@locked will be set to 0 when mmap_lock is
+ * released by an up_read().  That can happen if @gup_flags does not
+ * have FOLL_NOWAIT.
+ *
+ * A caller using such a combination of @locked and @gup_flags
+ * must therefore hold the mmap_lock for reading only, and recognize
+ * when it's been released.  Otherwise, it must be held for either
+ * reading or writing and will not be released.
+ *
+ * In most cases, get_user_pages or get_user_pages_fast should be used
+ * instead of __get_user_pages. __get_user_pages should be used only if
+ * you need some special @gup_flags.
+ */
+static long __get_user_pages(struct mm_struct *mm,
+		unsigned long start, unsigned long nr_pages,
+		unsigned int gup_flags, struct page **pages,
+		struct vm_area_struct **vmas, int *locked)
+{
+	long ret = 0, i = 0;
+	struct vm_area_struct *vma = NULL;
+	struct follow_page_context ctx = { NULL };
+
+	if (!nr_pages)
+		return 0;
+
+	start = untagged_addr(start);
+
+	VM_BUG_ON(!!pages != !!(gup_flags & (FOLL_GET | FOLL_PIN)));
+
+	/*
+	 * If FOLL_FORCE is set then do not force a full fault as the hinting
+	 * fault information is unrelated to the reference behaviour of a task
+	 * using the address space
+	 */
+	if (!(gup_flags & FOLL_FORCE))
+		gup_flags |= FOLL_NUMA;
+
+	do {
+		struct page *page;
+		unsigned int foll_flags = gup_flags;
+		unsigned int page_increm;
+
+		/* first iteration or cross vma bound */
+		if (!vma || start >= vma->vm_end) {
+			vma = find_extend_vma(mm, start);
+			if (!vma && in_gate_area(mm, start)) {
+				ret = get_gate_page(mm, start & PAGE_MASK,
+						gup_flags, &vma,
+						pages ? &pages[i] : NULL);
+				if (ret)
+					goto out;
+				ctx.page_mask = 0;
+				goto next_page;
+			}
+
+			if (!vma || check_vma_flags(vma, gup_flags)) {
+				ret = -EFAULT;
+				goto out;
+			}
+			if (is_vm_hugetlb_page(vma)) {
+				i = follow_hugetlb_page(mm, vma, pages, vmas,
+						&start, &nr_pages, i,
+						gup_flags, locked);
+				if (locked && *locked == 0) {
+					/*
+					 * We've got a VM_FAULT_RETRY
+					 * and we've lost mmap_lock.
+					 * We must stop here.
+					 */
+					BUG_ON(gup_flags & FOLL_NOWAIT);
+					BUG_ON(ret != 0);
+					goto out;
+				}
+				continue;
+			}
+		}
+retry:
+		/*
+		 * If we have a pending SIGKILL, don't keep faulting pages and
+		 * potentially allocating memory.
+		 */
+		if (fatal_signal_pending(current)) {
+			ret = -EINTR;
+			goto out;
+		}
+		cond_resched();
+
+		page = follow_page_mask(vma, start, foll_flags, &ctx);
+		if (!page) {
+			ret = faultin_page(vma, start, &foll_flags, locked);
+			switch (ret) {
+			case 0:
+				goto retry;
+			case -EBUSY:
+				ret = 0;
+				fallthrough;
+			case -EFAULT:
+			case -ENOMEM:
+			case -EHWPOISON:
+				goto out;
+			case -ENOENT:
+				goto next_page;
+			}
+			BUG();
+		} else if (PTR_ERR(page) == -EEXIST) {
+			/*
+			 * Proper page table entry exists, but no corresponding
+			 * struct page.
+			 */
+			goto next_page;
+		} else if (IS_ERR(page)) {
+			ret = PTR_ERR(page);
+			goto out;
+		}
+		if (pages) {
+			pages[i] = page;
+			flush_anon_page(vma, page, start);
+			flush_dcache_page(page);
+			ctx.page_mask = 0;
+		}
+next_page:
+		if (vmas) {
+			vmas[i] = vma;
+			ctx.page_mask = 0;
+		}
+		page_increm = 1 + (~(start >> PAGE_SHIFT) & ctx.page_mask);
+		if (page_increm > nr_pages)
+			page_increm = nr_pages;
+		i += page_increm;
+		start += page_increm * PAGE_SIZE;
+		nr_pages -= page_increm;
+	} while (nr_pages);
+out:
+	if (ctx.pgmap)
+		put_dev_pagemap(ctx.pgmap);
+	return i ? i : ret;
+}
+
+static bool vma_permits_fault(struct vm_area_struct *vma,
+			      unsigned int fault_flags)
+{
+	bool write   = !!(fault_flags & FAULT_FLAG_WRITE);
+	bool foreign = !!(fault_flags & FAULT_FLAG_REMOTE);
+	vm_flags_t vm_flags = write ? VM_WRITE : VM_READ;
+
+	if (!(vm_flags & vma->vm_flags))
+		return false;
+
+	/*
+	 * The architecture might have a hardware protection
+	 * mechanism other than read/write that can deny access.
+	 *
+	 * gup always represents data access, not instruction
+	 * fetches, so execute=false here:
+	 */
+	if (!arch_vma_access_permitted(vma, write, false, foreign))
+		return false;
+
+	return true;
+}
+
+/**
+ * fixup_user_fault() - manually resolve a user page fault
+ * @mm:		mm_struct of target mm
+ * @address:	user address
+ * @fault_flags:flags to pass down to handle_mm_fault()
+ * @unlocked:	did we unlock the mmap_lock while retrying, maybe NULL if caller
+ *		does not allow retry. If NULL, the caller must guarantee
+ *		that fault_flags does not contain FAULT_FLAG_ALLOW_RETRY.
+ *
+ * This is meant to be called in the specific scenario where for locking reasons
+ * we try to access user memory in atomic context (within a pagefault_disable()
+ * section), this returns -EFAULT, and we want to resolve the user fault before
+ * trying again.
+ *
+ * Typically this is meant to be used by the futex code.
+ *
+ * The main difference with get_user_pages() is that this function will
+ * unconditionally call handle_mm_fault() which will in turn perform all the
+ * necessary SW fixup of the dirty and young bits in the PTE, while
+ * get_user_pages() only guarantees to update these in the struct page.
+ *
+ * This is important for some architectures where those bits also gate the
+ * access permission to the page because they are maintained in software.  On
+ * such architectures, gup() will not be enough to make a subsequent access
+ * succeed.
+ *
+ * This function will not return with an unlocked mmap_lock. So it has not the
+ * same semantics wrt the @mm->mmap_lock as does filemap_fault().
+ */
+int fixup_user_fault(struct mm_struct *mm,
+		     unsigned long address, unsigned int fault_flags,
+		     bool *unlocked)
+{
+	struct vm_area_struct *vma;
+	vm_fault_t ret, major = 0;
+
+	address = untagged_addr(address);
+
+	if (unlocked)
+		fault_flags |= FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
+
+retry:
+	vma = find_extend_vma(mm, address);
+	if (!vma || address < vma->vm_start)
+		return -EFAULT;
+
+	if (!vma_permits_fault(vma, fault_flags))
+		return -EFAULT;
+
+	if ((fault_flags & FAULT_FLAG_KILLABLE) &&
+	    fatal_signal_pending(current))
+		return -EINTR;
+
+	ret = handle_mm_fault(vma, address, fault_flags, NULL);
+	major |= ret & VM_FAULT_MAJOR;
+	if (ret & VM_FAULT_ERROR) {
+		int err = vm_fault_to_errno(ret, 0);
+
+		if (err)
+			return err;
+		BUG();
+	}
+
+	if (ret & VM_FAULT_RETRY) {
+		mmap_read_lock(mm);
+		*unlocked = true;
+		fault_flags |= FAULT_FLAG_TRIED;
+		goto retry;
+	}
+
+	return 0;
+}
+EXPORT_SYMBOL_GPL(fixup_user_fault);
+
+/*
+ * Please note that this function, unlike __get_user_pages will not
+ * return 0 for nr_pages > 0 without FOLL_NOWAIT
+ */
+static __always_inline long __get_user_pages_locked(struct mm_struct *mm,
+						unsigned long start,
+						unsigned long nr_pages,
+						struct page **pages,
+						struct vm_area_struct **vmas,
+						int *locked,
+						unsigned int flags)
+{
+	long ret, pages_done;
+	bool lock_dropped;
+
+	if (locked) {
+		/* if VM_FAULT_RETRY can be returned, vmas become invalid */
+		BUG_ON(vmas);
+		/* check caller initialized locked */
+		BUG_ON(*locked != 1);
+	}
+
+	if (flags & FOLL_PIN)
+		atomic_set(&mm->has_pinned, 1);
+
+	/*
+	 * FOLL_PIN and FOLL_GET are mutually exclusive. Traditional behavior
+	 * is to set FOLL_GET if the caller wants pages[] filled in (but has
+	 * carelessly failed to specify FOLL_GET), so keep doing that, but only
+	 * for FOLL_GET, not for the newer FOLL_PIN.
+	 *
+	 * FOLL_PIN always expects pages to be non-null, but no need to assert
+	 * that here, as any failures will be obvious enough.
+	 */
+	if (pages && !(flags & FOLL_PIN))
+		flags |= FOLL_GET;
+
+	pages_done = 0;
+	lock_dropped = false;
+	for (;;) {
+		ret = __get_user_pages(mm, start, nr_pages, flags, pages,
+				       vmas, locked);
+		if (!locked)
+			/* VM_FAULT_RETRY couldn't trigger, bypass */
+			return ret;
+
+		/* VM_FAULT_RETRY cannot return errors */
+		if (!*locked) {
+			BUG_ON(ret < 0);
+			BUG_ON(ret >= nr_pages);
+		}
+
+		if (ret > 0) {
+			nr_pages -= ret;
+			pages_done += ret;
+			if (!nr_pages)
+				break;
+		}
+		if (*locked) {
+			/*
+			 * VM_FAULT_RETRY didn't trigger or it was a
+			 * FOLL_NOWAIT.
+			 */
+			if (!pages_done)
+				pages_done = ret;
+			break;
+		}
+		/*
+		 * VM_FAULT_RETRY triggered, so seek to the faulting offset.
+		 * For the prefault case (!pages) we only update counts.
+		 */
+		if (likely(pages))
+			pages += ret;
+		start += ret << PAGE_SHIFT;
+		lock_dropped = true;
+
+retry:
+		/*
+		 * Repeat on the address that fired VM_FAULT_RETRY
+		 * with both FAULT_FLAG_ALLOW_RETRY and
+		 * FAULT_FLAG_TRIED.  Note that GUP can be interrupted
+		 * by fatal signals, so we need to check it before we
+		 * start trying again otherwise it can loop forever.
+		 */
+
+		if (fatal_signal_pending(current)) {
+			if (!pages_done)
+				pages_done = -EINTR;
+			break;
+		}
+
+		ret = mmap_read_lock_killable(mm);
+		if (ret) {
+			BUG_ON(ret > 0);
+			if (!pages_done)
+				pages_done = ret;
+			break;
+		}
+
+		*locked = 1;
+		ret = __get_user_pages(mm, start, 1, flags | FOLL_TRIED,
+				       pages, NULL, locked);
+		if (!*locked) {
+			/* Continue to retry until we succeeded */
+			BUG_ON(ret != 0);
+			goto retry;
+		}
+		if (ret != 1) {
+			BUG_ON(ret > 1);
+			if (!pages_done)
+				pages_done = ret;
+			break;
+		}
+		nr_pages--;
+		pages_done++;
+		if (!nr_pages)
+			break;
+		if (likely(pages))
+			pages++;
+		start += PAGE_SIZE;
+	}
+	if (lock_dropped && *locked) {
+		/*
+		 * We must let the caller know we temporarily dropped the lock
+		 * and so the critical section protected by it was lost.
+		 */
+		mmap_read_unlock(mm);
+		*locked = 0;
+	}
+	return pages_done;
+}
+
+/**
+ * populate_vma_page_range() -  populate a range of pages in the vma.
+ * @vma:   target vma
+ * @start: start address
+ * @end:   end address
+ * @locked: whether the mmap_lock is still held
+ *
+ * This takes care of mlocking the pages too if VM_LOCKED is set.
+ *
+ * Return either number of pages pinned in the vma, or a negative error
+ * code on error.
+ *
+ * vma->vm_mm->mmap_lock must be held.
+ *
+ * If @locked is NULL, it may be held for read or write and will
+ * be unperturbed.
+ *
+ * If @locked is non-NULL, it must held for read only and may be
+ * released.  If it's released, *@locked will be set to 0.
+ */
+long populate_vma_page_range(struct vm_area_struct *vma,
+		unsigned long start, unsigned long end, int *locked)
+{
+	struct mm_struct *mm = vma->vm_mm;
+	unsigned long nr_pages = (end - start) / PAGE_SIZE;
+	int gup_flags;
+
+	VM_BUG_ON(start & ~PAGE_MASK);
+	VM_BUG_ON(end   & ~PAGE_MASK);
+	VM_BUG_ON_VMA(start < vma->vm_start, vma);
+	VM_BUG_ON_VMA(end   > vma->vm_end, vma);
+	mmap_assert_locked(mm);
+
+	gup_flags = FOLL_TOUCH | FOLL_POPULATE | FOLL_MLOCK;
+	if (vma->vm_flags & VM_LOCKONFAULT)
+		gup_flags &= ~FOLL_POPULATE;
+	/*
+	 * We want to touch writable mappings with a write fault in order
+	 * to break COW, except for shared mappings because these don't COW
+	 * and we would not want to dirty them for nothing.
+	 */
+	if ((vma->vm_flags & (VM_WRITE | VM_SHARED)) == VM_WRITE)
+		gup_flags |= FOLL_WRITE;
+
+	/*
+	 * We want mlock to succeed for regions that have any permissions
+	 * other than PROT_NONE.
+	 */
+	if (vma_is_accessible(vma))
+		gup_flags |= FOLL_FORCE;
+
+	/*
+	 * We made sure addr is within a VMA, so the following will
+	 * not result in a stack expansion that recurses back here.
+	 */
+	return __get_user_pages(mm, start, nr_pages, gup_flags,
+				NULL, NULL, locked);
+}
+
+/*
+ * __mm_populate - populate and/or mlock pages within a range of address space.
+ *
+ * This is used to implement mlock() and the MAP_POPULATE / MAP_LOCKED mmap
+ * flags. VMAs must be already marked with the desired vm_flags, and
+ * mmap_lock must not be held.
+ */
+int __mm_populate(unsigned long start, unsigned long len, int ignore_errors)
+{
+	struct mm_struct *mm = current->mm;
+	unsigned long end, nstart, nend;
+	struct vm_area_struct *vma = NULL;
+	int locked = 0;
+	long ret = 0;
+
+	end = start + len;
+
+	for (nstart = start; nstart < end; nstart = nend) {
+		/*
+		 * We want to fault in pages for [nstart; end) address range.
+		 * Find first corresponding VMA.
+		 */
+		if (!locked) {
+			locked = 1;
+			mmap_read_lock(mm);
+			vma = find_vma(mm, nstart);
+		} else if (nstart >= vma->vm_end)
+			vma = vma->vm_next;
+		if (!vma || vma->vm_start >= end)
+			break;
+		/*
+		 * Set [nstart; nend) to intersection of desired address
+		 * range with the first VMA. Also, skip undesirable VMA types.
+		 */
+		nend = min(end, vma->vm_end);
+		if (vma->vm_flags & (VM_IO | VM_PFNMAP))
+			continue;
+		if (nstart < vma->vm_start)
+			nstart = vma->vm_start;
+		/*
+		 * Now fault in a range of pages. populate_vma_page_range()
+		 * double checks the vma flags, so that it won't mlock pages
+		 * if the vma was already munlocked.
+		 */
+		ret = populate_vma_page_range(vma, nstart, nend, &locked);
+		if (ret < 0) {
+			if (ignore_errors) {
+				ret = 0;
+				continue;	/* continue at next VMA */
+			}
+			break;
+		}
+		nend = nstart + ret * PAGE_SIZE;
+		ret = 0;
+	}
+	if (locked)
+		mmap_read_unlock(mm);
+	return ret;	/* 0 or negative error code */
+}
+#else /* CONFIG_MMU */
+static long __get_user_pages_locked(struct mm_struct *mm, unsigned long start,
+		unsigned long nr_pages, struct page **pages,
+		struct vm_area_struct **vmas, int *locked,
+		unsigned int foll_flags)
+{
+	struct vm_area_struct *vma;
+	unsigned long vm_flags;
+	int i;
+
+	/* calculate required read or write permissions.
+	 * If FOLL_FORCE is set, we only require the "MAY" flags.
+	 */
+	vm_flags  = (foll_flags & FOLL_WRITE) ?
+			(VM_WRITE | VM_MAYWRITE) : (VM_READ | VM_MAYREAD);
+	vm_flags &= (foll_flags & FOLL_FORCE) ?
+			(VM_MAYREAD | VM_MAYWRITE) : (VM_READ | VM_WRITE);
+
+	for (i = 0; i < nr_pages; i++) {
+		vma = find_vma(mm, start);
+		if (!vma)
+			goto finish_or_fault;
+
+		/* protect what we can, including chardevs */
+		if ((vma->vm_flags & (VM_IO | VM_PFNMAP)) ||
+		    !(vm_flags & vma->vm_flags))
+			goto finish_or_fault;
+
+		if (pages) {
+			pages[i] = virt_to_page(start);
+			if (pages[i])
+				get_page(pages[i]);
+		}
+		if (vmas)
+			vmas[i] = vma;
+		start = (start + PAGE_SIZE) & PAGE_MASK;
+	}
+
+	return i;
+
+finish_or_fault:
+	return i ? : -EFAULT;
+}
+#endif /* !CONFIG_MMU */
+
+/**
+ * get_dump_page() - pin user page in memory while writing it to core dump
+ * @addr: user address
+ *
+ * Returns struct page pointer of user page pinned for dump,
+ * to be freed afterwards by put_page().
+ *
+ * Returns NULL on any kind of failure - a hole must then be inserted into
+ * the corefile, to preserve alignment with its headers; and also returns
+ * NULL wherever the ZERO_PAGE, or an anonymous pte_none, has been found -
+ * allowing a hole to be left in the corefile to save diskspace.
+ *
+ * Called without mmap_lock (takes and releases the mmap_lock by itself).
+ */
+#ifdef CONFIG_ELF_CORE
+struct page *get_dump_page(unsigned long addr)
+{
+	struct mm_struct *mm = current->mm;
+	struct page *page;
+	int locked = 1;
+	int ret;
+
+	if (mmap_read_lock_killable(mm))
+		return NULL;
+	ret = __get_user_pages_locked(mm, addr, 1, &page, NULL, &locked,
+				      FOLL_FORCE | FOLL_DUMP | FOLL_GET);
+	if (locked)
+		mmap_read_unlock(mm);
+	return (ret == 1) ? page : NULL;
+}
+#endif /* CONFIG_ELF_CORE */
+
+#if defined(CONFIG_FS_DAX) || defined (CONFIG_CMA)
+static bool check_dax_vmas(struct vm_area_struct **vmas, long nr_pages)
+{
+	long i;
+	struct vm_area_struct *vma_prev = NULL;
+
+	for (i = 0; i < nr_pages; i++) {
+		struct vm_area_struct *vma = vmas[i];
+
+		if (vma == vma_prev)
+			continue;
+
+		vma_prev = vma;
+
+		if (vma_is_fsdax(vma))
+			return true;
+	}
+	return false;
+}
+
+#ifdef CONFIG_CMA
+static long check_and_migrate_cma_pages(struct mm_struct *mm,
+					unsigned long start,
+					unsigned long nr_pages,
+					struct page **pages,
+					struct vm_area_struct **vmas,
+					unsigned int gup_flags)
+{
+	unsigned long i, isolation_error_count;
+	bool drain_allow;
+	LIST_HEAD(cma_page_list);
+	long ret = nr_pages;
+	struct page *prev_head, *head;
+	struct migration_target_control mtc = {
+		.nid = NUMA_NO_NODE,
+		.gfp_mask = GFP_USER | __GFP_MOVABLE | __GFP_NOWARN,
+	};
+
+check_again:
+	prev_head = NULL;
+	isolation_error_count = 0;
+	drain_allow = true;
+	for (i = 0; i < nr_pages; i++) {
+		head = compound_head(pages[i]);
+		if (head == prev_head)
+			continue;
+		prev_head = head;
+		/*
+		 * If we get a page from the CMA zone, since we are going to
+		 * be pinning these entries, we might as well move them out
+		 * of the CMA zone if possible.
+		 */
+		if (is_migrate_cma_page(head)) {
+			if (PageHuge(head)) {
+				if (isolate_hugetlb(head, &cma_page_list))
+					isolation_error_count++;
+			} else {
+				if (!PageLRU(head) && drain_allow) {
+					lru_add_drain_all();
+					drain_allow = false;
+				}
+
+				if (isolate_lru_page(head)) {
+					isolation_error_count++;
+					continue;
+				}
+				list_add_tail(&head->lru, &cma_page_list);
+				mod_node_page_state(page_pgdat(head),
+						    NR_ISOLATED_ANON +
+						    page_is_file_lru(head),
+						    thp_nr_pages(head));
+			}
+		}
+	}
+
+	/*
+	 * If list is empty, and no isolation errors, means that all pages are
+	 * in the correct zone.
+	 */
+	if (list_empty(&cma_page_list) && !isolation_error_count)
+		return ret;
+
+	if (!list_empty(&cma_page_list)) {
+		/*
+		 * drop the above get_user_pages reference.
+		 */
+		if (gup_flags & FOLL_PIN)
+			unpin_user_pages(pages, nr_pages);
+		else
+			for (i = 0; i < nr_pages; i++)
+				put_page(pages[i]);
+
+		ret = migrate_pages(&cma_page_list, alloc_migration_target,
+				    NULL, (unsigned long)&mtc, MIGRATE_SYNC,
+				    MR_CONTIG_RANGE);
+		if (ret) {
+			if (!list_empty(&cma_page_list))
+				putback_movable_pages(&cma_page_list);
+			return ret > 0 ? -ENOMEM : ret;
+		}
+
+		/* We unpinned pages before migration, pin them again */
+		ret = __get_user_pages_locked(mm, start, nr_pages, pages, vmas,
+					      NULL, gup_flags);
+		if (ret <= 0)
+			return ret;
+		nr_pages = ret;
+	}
+
+	/*
+	 * check again because pages were unpinned, and we also might have
+	 * had isolation errors and need more pages to migrate.
+	 */
+	goto check_again;
+}
+#else
+static long check_and_migrate_cma_pages(struct mm_struct *mm,
+					unsigned long start,
+					unsigned long nr_pages,
+					struct page **pages,
+					struct vm_area_struct **vmas,
+					unsigned int gup_flags)
+{
+	return nr_pages;
+}
+#endif /* CONFIG_CMA */
+
+/*
+ * __gup_longterm_locked() is a wrapper for __get_user_pages_locked which
+ * allows us to process the FOLL_LONGTERM flag.
+ */
+static long __gup_longterm_locked(struct mm_struct *mm,
+				  unsigned long start,
+				  unsigned long nr_pages,
+				  struct page **pages,
+				  struct vm_area_struct **vmas,
+				  unsigned int gup_flags)
+{
+	struct vm_area_struct **vmas_tmp = vmas;
+	unsigned long flags = 0;
+	long rc, i;
+
+	if (gup_flags & FOLL_LONGTERM) {
+		if (!pages)
+			return -EINVAL;
+
+		if (!vmas_tmp) {
+			vmas_tmp = kcalloc(nr_pages,
+					   sizeof(struct vm_area_struct *),
+					   GFP_KERNEL);
+			if (!vmas_tmp)
+				return -ENOMEM;
+		}
+		flags = memalloc_nocma_save();
+	}
+
+	rc = __get_user_pages_locked(mm, start, nr_pages, pages,
+				     vmas_tmp, NULL, gup_flags);
+
+	if (gup_flags & FOLL_LONGTERM) {
+		if (rc < 0)
+			goto out;
+
+		if (check_dax_vmas(vmas_tmp, rc)) {
+			if (gup_flags & FOLL_PIN)
+				unpin_user_pages(pages, rc);
+			else
+				for (i = 0; i < rc; i++)
+					put_page(pages[i]);
+			rc = -EOPNOTSUPP;
+			goto out;
+		}
+
+		rc = check_and_migrate_cma_pages(mm, start, rc, pages,
+						 vmas_tmp, gup_flags);
+out:
+		memalloc_nocma_restore(flags);
+	}
+
+	if (vmas_tmp != vmas)
+		kfree(vmas_tmp);
+	return rc;
+}
+#else /* !CONFIG_FS_DAX && !CONFIG_CMA */
+static __always_inline long __gup_longterm_locked(struct mm_struct *mm,
+						  unsigned long start,
+						  unsigned long nr_pages,
+						  struct page **pages,
+						  struct vm_area_struct **vmas,
+						  unsigned int flags)
+{
+	return __get_user_pages_locked(mm, start, nr_pages, pages, vmas,
+				       NULL, flags);
+}
+#endif /* CONFIG_FS_DAX || CONFIG_CMA */
+
+static bool is_valid_gup_flags(unsigned int gup_flags)
+{
+	/*
+	 * FOLL_PIN must only be set internally by the pin_user_pages*() APIs,
+	 * never directly by the caller, so enforce that with an assertion:
+	 */
+	if (WARN_ON_ONCE(gup_flags & FOLL_PIN))
+		return false;
+	/*
+	 * FOLL_PIN is a prerequisite to FOLL_LONGTERM. Another way of saying
+	 * that is, FOLL_LONGTERM is a specific case, more restrictive case of
+	 * FOLL_PIN.
+	 */
+	if (WARN_ON_ONCE(gup_flags & FOLL_LONGTERM))
+		return false;
+
+	return true;
+}
+
+#ifdef CONFIG_MMU
+static long __get_user_pages_remote(struct mm_struct *mm,
+				    unsigned long start, unsigned long nr_pages,
+				    unsigned int gup_flags, struct page **pages,
+				    struct vm_area_struct **vmas, int *locked)
+{
+	/*
+	 * Parts of FOLL_LONGTERM behavior are incompatible with
+	 * FAULT_FLAG_ALLOW_RETRY because of the FS DAX check requirement on
+	 * vmas. However, this only comes up if locked is set, and there are
+	 * callers that do request FOLL_LONGTERM, but do not set locked. So,
+	 * allow what we can.
+	 */
+	if (gup_flags & FOLL_LONGTERM) {
+		if (WARN_ON_ONCE(locked))
+			return -EINVAL;
+		/*
+		 * This will check the vmas (even if our vmas arg is NULL)
+		 * and return -ENOTSUPP if DAX isn't allowed in this case:
+		 */
+		return __gup_longterm_locked(mm, start, nr_pages, pages,
+					     vmas, gup_flags | FOLL_TOUCH |
+					     FOLL_REMOTE);
+	}
+
+	return __get_user_pages_locked(mm, start, nr_pages, pages, vmas,
+				       locked,
+				       gup_flags | FOLL_TOUCH | FOLL_REMOTE);
+}
+
+/**
+ * get_user_pages_remote() - pin user pages in memory
+ * @mm:		mm_struct of target mm
+ * @start:	starting user address
+ * @nr_pages:	number of pages from start to pin
+ * @gup_flags:	flags modifying lookup behaviour
+ * @pages:	array that receives pointers to the pages pinned.
+ *		Should be at least nr_pages long. Or NULL, if caller
+ *		only intends to ensure the pages are faulted in.
+ * @vmas:	array of pointers to vmas corresponding to each page.
+ *		Or NULL if the caller does not require them.
+ * @locked:	pointer to lock flag indicating whether lock is held and
+ *		subsequently whether VM_FAULT_RETRY functionality can be
+ *		utilised. Lock must initially be held.
+ *
+ * Returns either number of pages pinned (which may be less than the
+ * number requested), or an error. Details about the return value:
+ *
+ * -- If nr_pages is 0, returns 0.
+ * -- If nr_pages is >0, but no pages were pinned, returns -errno.
+ * -- If nr_pages is >0, and some pages were pinned, returns the number of
+ *    pages pinned. Again, this may be less than nr_pages.
+ *
+ * The caller is responsible for releasing returned @pages, via put_page().
+ *
+ * @vmas are valid only as long as mmap_lock is held.
+ *
+ * Must be called with mmap_lock held for read or write.
+ *
+ * get_user_pages_remote walks a process's page tables and takes a reference
+ * to each struct page that each user address corresponds to at a given
+ * instant. That is, it takes the page that would be accessed if a user
+ * thread accesses the given user virtual address at that instant.
+ *
+ * This does not guarantee that the page exists in the user mappings when
+ * get_user_pages_remote returns, and there may even be a completely different
+ * page there in some cases (eg. if mmapped pagecache has been invalidated
+ * and subsequently re faulted). However it does guarantee that the page
+ * won't be freed completely. And mostly callers simply care that the page
+ * contains data that was valid *at some point in time*. Typically, an IO
+ * or similar operation cannot guarantee anything stronger anyway because
+ * locks can't be held over the syscall boundary.
+ *
+ * If gup_flags & FOLL_WRITE == 0, the page must not be written to. If the page
+ * is written to, set_page_dirty (or set_page_dirty_lock, as appropriate) must
+ * be called after the page is finished with, and before put_page is called.
+ *
+ * get_user_pages_remote is typically used for fewer-copy IO operations,
+ * to get a handle on the memory by some means other than accesses
+ * via the user virtual addresses. The pages may be submitted for
+ * DMA to devices or accessed via their kernel linear mapping (via the
+ * kmap APIs). Care should be taken to use the correct cache flushing APIs.
+ *
+ * See also get_user_pages_fast, for performance critical applications.
+ *
+ * get_user_pages_remote should be phased out in favor of
+ * get_user_pages_locked|unlocked or get_user_pages_fast. Nothing
+ * should use get_user_pages_remote because it cannot pass
+ * FAULT_FLAG_ALLOW_RETRY to handle_mm_fault.
+ */
+long get_user_pages_remote(struct mm_struct *mm,
+		unsigned long start, unsigned long nr_pages,
+		unsigned int gup_flags, struct page **pages,
+		struct vm_area_struct **vmas, int *locked)
+{
+	if (!is_valid_gup_flags(gup_flags))
+		return -EINVAL;
+
+	return __get_user_pages_remote(mm, start, nr_pages, gup_flags,
+				       pages, vmas, locked);
+}
+EXPORT_SYMBOL(get_user_pages_remote);
+
+#else /* CONFIG_MMU */
+long get_user_pages_remote(struct mm_struct *mm,
+			   unsigned long start, unsigned long nr_pages,
+			   unsigned int gup_flags, struct page **pages,
+			   struct vm_area_struct **vmas, int *locked)
+{
+	return 0;
+}
+
+static long __get_user_pages_remote(struct mm_struct *mm,
+				    unsigned long start, unsigned long nr_pages,
+				    unsigned int gup_flags, struct page **pages,
+				    struct vm_area_struct **vmas, int *locked)
+{
+	return 0;
+}
+#endif /* !CONFIG_MMU */
+
+/**
+ * get_user_pages() - pin user pages in memory
+ * @start:      starting user address
+ * @nr_pages:   number of pages from start to pin
+ * @gup_flags:  flags modifying lookup behaviour
+ * @pages:      array that receives pointers to the pages pinned.
+ *              Should be at least nr_pages long. Or NULL, if caller
+ *              only intends to ensure the pages are faulted in.
+ * @vmas:       array of pointers to vmas corresponding to each page.
+ *              Or NULL if the caller does not require them.
+ *
+ * This is the same as get_user_pages_remote(), just with a less-flexible
+ * calling convention where we assume that the mm being operated on belongs to
+ * the current task, and doesn't allow passing of a locked parameter.  We also
+ * obviously don't pass FOLL_REMOTE in here.
+ */
+long get_user_pages(unsigned long start, unsigned long nr_pages,
+		unsigned int gup_flags, struct page **pages,
+		struct vm_area_struct **vmas)
+{
+	if (!is_valid_gup_flags(gup_flags))
+		return -EINVAL;
+
+	return __gup_longterm_locked(current->mm, start, nr_pages,
+				     pages, vmas, gup_flags | FOLL_TOUCH);
+}
+EXPORT_SYMBOL(get_user_pages);
+
+/**
+ * get_user_pages_locked() is suitable to replace the form:
+ *
+ *      mmap_read_lock(mm);
+ *      do_something()
+ *      get_user_pages(mm, ..., pages, NULL);
+ *      mmap_read_unlock(mm);
+ *
+ *  to:
+ *
+ *      int locked = 1;
+ *      mmap_read_lock(mm);
+ *      do_something()
+ *      get_user_pages_locked(mm, ..., pages, &locked);
+ *      if (locked)
+ *          mmap_read_unlock(mm);
+ *
+ * @start:      starting user address
+ * @nr_pages:   number of pages from start to pin
+ * @gup_flags:  flags modifying lookup behaviour
+ * @pages:      array that receives pointers to the pages pinned.
+ *              Should be at least nr_pages long. Or NULL, if caller
+ *              only intends to ensure the pages are faulted in.
+ * @locked:     pointer to lock flag indicating whether lock is held and
+ *              subsequently whether VM_FAULT_RETRY functionality can be
+ *              utilised. Lock must initially be held.
+ *
+ * We can leverage the VM_FAULT_RETRY functionality in the page fault
+ * paths better by using either get_user_pages_locked() or
+ * get_user_pages_unlocked().
+ *
+ */
+long get_user_pages_locked(unsigned long start, unsigned long nr_pages,
+			   unsigned int gup_flags, struct page **pages,
+			   int *locked)
+{
+	/*
+	 * FIXME: Current FOLL_LONGTERM behavior is incompatible with
+	 * FAULT_FLAG_ALLOW_RETRY because of the FS DAX check requirement on
+	 * vmas.  As there are no users of this flag in this call we simply
+	 * disallow this option for now.
+	 */
+	if (WARN_ON_ONCE(gup_flags & FOLL_LONGTERM))
+		return -EINVAL;
+	/*
+	 * FOLL_PIN must only be set internally by the pin_user_pages*() APIs,
+	 * never directly by the caller, so enforce that:
+	 */
+	if (WARN_ON_ONCE(gup_flags & FOLL_PIN))
+		return -EINVAL;
+
+	return __get_user_pages_locked(current->mm, start, nr_pages,
+				       pages, NULL, locked,
+				       gup_flags | FOLL_TOUCH);
+}
+EXPORT_SYMBOL(get_user_pages_locked);
+
+/*
+ * get_user_pages_unlocked() is suitable to replace the form:
+ *
+ *      mmap_read_lock(mm);
+ *      get_user_pages(mm, ..., pages, NULL);
+ *      mmap_read_unlock(mm);
+ *
+ *  with:
+ *
+ *      get_user_pages_unlocked(mm, ..., pages);
+ *
+ * It is functionally equivalent to get_user_pages_fast so
+ * get_user_pages_fast should be used instead if specific gup_flags
+ * (e.g. FOLL_FORCE) are not required.
+ */
+long get_user_pages_unlocked(unsigned long start, unsigned long nr_pages,
+			     struct page **pages, unsigned int gup_flags)
+{
+	struct mm_struct *mm = current->mm;
+	int locked = 1;
+	long ret;
+
+	/*
+	 * FIXME: Current FOLL_LONGTERM behavior is incompatible with
+	 * FAULT_FLAG_ALLOW_RETRY because of the FS DAX check requirement on
+	 * vmas.  As there are no users of this flag in this call we simply
+	 * disallow this option for now.
+	 */
+	if (WARN_ON_ONCE(gup_flags & FOLL_LONGTERM))
+		return -EINVAL;
+
+	mmap_read_lock(mm);
+	ret = __get_user_pages_locked(mm, start, nr_pages, pages, NULL,
+				      &locked, gup_flags | FOLL_TOUCH);
+	if (locked)
+		mmap_read_unlock(mm);
+	return ret;
+}
+EXPORT_SYMBOL(get_user_pages_unlocked);
+
+/*
+ * Fast GUP
+ *
+ * get_user_pages_fast attempts to pin user pages by walking the page
+ * tables directly and avoids taking locks. Thus the walker needs to be
+ * protected from page table pages being freed from under it, and should
+ * block any THP splits.
+ *
+ * One way to achieve this is to have the walker disable interrupts, and
+ * rely on IPIs from the TLB flushing code blocking before the page table
+ * pages are freed. This is unsuitable for architectures that do not need
+ * to broadcast an IPI when invalidating TLBs.
+ *
+ * Another way to achieve this is to batch up page table containing pages
+ * belonging to more than one mm_user, then rcu_sched a callback to free those
+ * pages. Disabling interrupts will allow the fast_gup walker to both block
+ * the rcu_sched callback, and an IPI that we broadcast for splitting THPs
+ * (which is a relatively rare event). The code below adopts this strategy.
+ *
+ * Before activating this code, please be aware that the following assumptions
+ * are currently made:
+ *
+ *  *) Either MMU_GATHER_RCU_TABLE_FREE is enabled, and tlb_remove_table() is used to
+ *  free pages containing page tables or TLB flushing requires IPI broadcast.
+ *
+ *  *) ptes can be read atomically by the architecture.
+ *
+ *  *) access_ok is sufficient to validate userspace address ranges.
+ *
+ * The last two assumptions can be relaxed by the addition of helper functions.
+ *
+ * This code is based heavily on the PowerPC implementation by Nick Piggin.
+ */
+#ifdef CONFIG_HAVE_FAST_GUP
+#ifdef CONFIG_GUP_GET_PTE_LOW_HIGH
+
+/*
+ * WARNING: only to be used in the get_user_pages_fast() implementation.
+ *
+ * With get_user_pages_fast(), we walk down the pagetables without taking any
+ * locks.  For this we would like to load the pointers atomically, but sometimes
+ * that is not possible (e.g. without expensive cmpxchg8b on x86_32 PAE).  What
+ * we do have is the guarantee that a PTE will only either go from not present
+ * to present, or present to not present or both -- it will not switch to a
+ * completely different present page without a TLB flush in between; something
+ * that we are blocking by holding interrupts off.
+ *
+ * Setting ptes from not present to present goes:
+ *
+ *   ptep->pte_high = h;
+ *   smp_wmb();
+ *   ptep->pte_low = l;
+ *
+ * And present to not present goes:
+ *
+ *   ptep->pte_low = 0;
+ *   smp_wmb();
+ *   ptep->pte_high = 0;
+ *
+ * We must ensure here that the load of pte_low sees 'l' IFF pte_high sees 'h'.
+ * We load pte_high *after* loading pte_low, which ensures we don't see an older
+ * value of pte_high.  *Then* we recheck pte_low, which ensures that we haven't
+ * picked up a changed pte high. We might have gotten rubbish values from
+ * pte_low and pte_high, but we are guaranteed that pte_low will not have the
+ * present bit set *unless* it is 'l'. Because get_user_pages_fast() only
+ * operates on present ptes we're safe.
+ */
+static inline pte_t gup_get_pte(pte_t *ptep)
+{
+	pte_t pte;
+
+	do {
+		pte.pte_low = ptep->pte_low;
+		smp_rmb();
+		pte.pte_high = ptep->pte_high;
+		smp_rmb();
+	} while (unlikely(pte.pte_low != ptep->pte_low));
+
+	return pte;
+}
+#else /* CONFIG_GUP_GET_PTE_LOW_HIGH */
+/*
+ * We require that the PTE can be read atomically.
+ */
+static inline pte_t gup_get_pte(pte_t *ptep)
+{
+	return ptep_get(ptep);
+}
+#endif /* CONFIG_GUP_GET_PTE_LOW_HIGH */
+
+static void __maybe_unused undo_dev_pagemap(int *nr, int nr_start,
+					    unsigned int flags,
+					    struct page **pages)
+{
+	while ((*nr) - nr_start) {
+		struct page *page = pages[--(*nr)];
+
+		ClearPageReferenced(page);
+		if (flags & FOLL_PIN)
+			unpin_user_page(page);
+		else
+			put_page(page);
+	}
+}
+
+#ifdef CONFIG_ARCH_HAS_PTE_SPECIAL
+/*
+ * Fast-gup relies on pte change detection to avoid concurrent pgtable
+ * operations.
+ *
+ * To pin the page, fast-gup needs to do below in order:
+ * (1) pin the page (by prefetching pte), then (2) check pte not changed.
+ *
+ * For the rest of pgtable operations where pgtable updates can be racy
+ * with fast-gup, we need to do (1) clear pte, then (2) check whether page
+ * is pinned.
+ *
+ * Above will work for all pte-level operations, including THP split.
+ *
+ * For THP collapse, it's a bit more complicated because fast-gup may be
+ * walking a pgtable page that is being freed (pte is still valid but pmd
+ * can be cleared already).  To avoid race in such condition, we need to
+ * also check pmd here to make sure pmd doesn't change (corresponds to
+ * pmdp_collapse_flush() in the THP collapse code path).
+ */
+static int gup_pte_range(pmd_t pmd, pmd_t *pmdp, unsigned long addr,
+			 unsigned long end, unsigned int flags,
+			 struct page **pages, int *nr)
+{
+	struct dev_pagemap *pgmap = NULL;
+	int nr_start = *nr, ret = 0;
+	pte_t *ptep, *ptem;
+
+	ptem = ptep = pte_offset_map(&pmd, addr);
+	do {
+		pte_t pte = gup_get_pte(ptep);
+		struct page *head, *page;
+
+		/*
+		 * Similar to the PMD case below, NUMA hinting must take slow
+		 * path using the pte_protnone check.
+		 */
+		if (pte_protnone(pte))
+			goto pte_unmap;
+
+		if (!pte_access_permitted(pte, flags & FOLL_WRITE))
+			goto pte_unmap;
+
+		if (pte_devmap(pte)) {
+			if (unlikely(flags & FOLL_LONGTERM))
+				goto pte_unmap;
+
+			pgmap = get_dev_pagemap(pte_pfn(pte), pgmap);
+			if (unlikely(!pgmap)) {
+				undo_dev_pagemap(nr, nr_start, flags, pages);
+				goto pte_unmap;
+			}
+		} else if (pte_special(pte))
+			goto pte_unmap;
+
+		VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
+		page = pte_page(pte);
+
+		head = try_grab_compound_head(page, 1, flags);
+		if (!head)
+			goto pte_unmap;
+
+		if (unlikely(pmd_val(pmd) != pmd_val(*pmdp)) ||
+		    unlikely(pte_val(pte) != pte_val(*ptep))) {
+			put_compound_head(head, 1, flags);
+			goto pte_unmap;
+		}
+
+		VM_BUG_ON_PAGE(compound_head(page) != head, page);
+
+		/*
+		 * We need to make the page accessible if and only if we are
+		 * going to access its content (the FOLL_PIN case).  Please
+		 * see Documentation/core-api/pin_user_pages.rst for
+		 * details.
+		 */
+		if (flags & FOLL_PIN) {
+			ret = arch_make_page_accessible(page);
+			if (ret) {
+				unpin_user_page(page);
+				goto pte_unmap;
+			}
+		}
+		SetPageReferenced(page);
+		pages[*nr] = page;
+		(*nr)++;
+
+	} while (ptep++, addr += PAGE_SIZE, addr != end);
+
+	ret = 1;
+
+pte_unmap:
+	if (pgmap)
+		put_dev_pagemap(pgmap);
+	pte_unmap(ptem);
+	return ret;
+}
+#else
+
+/*
+ * If we can't determine whether or not a pte is special, then fail immediately
+ * for ptes. Note, we can still pin HugeTLB and THP as these are guaranteed not
+ * to be special.
+ *
+ * For a futex to be placed on a THP tail page, get_futex_key requires a
+ * get_user_pages_fast_only implementation that can pin pages. Thus it's still
+ * useful to have gup_huge_pmd even if we can't operate on ptes.
+ */
+static int gup_pte_range(pmd_t pmd, pmd_t *pmdp, unsigned long addr,
+			 unsigned long end, unsigned int flags,
+			 struct page **pages, int *nr)
+{
+	return 0;
+}
+#endif /* CONFIG_ARCH_HAS_PTE_SPECIAL */
+
+#if defined(CONFIG_ARCH_HAS_PTE_DEVMAP) && defined(CONFIG_TRANSPARENT_HUGEPAGE)
+static int __gup_device_huge(unsigned long pfn, unsigned long addr,
+			     unsigned long end, unsigned int flags,
+			     struct page **pages, int *nr)
+{
+	int nr_start = *nr;
+	struct dev_pagemap *pgmap = NULL;
+
+	do {
+		struct page *page = pfn_to_page(pfn);
+
+		pgmap = get_dev_pagemap(pfn, pgmap);
+		if (unlikely(!pgmap)) {
+			undo_dev_pagemap(nr, nr_start, flags, pages);
+			return 0;
+		}
+		SetPageReferenced(page);
+		pages[*nr] = page;
+		if (unlikely(!try_grab_page(page, flags))) {
+			undo_dev_pagemap(nr, nr_start, flags, pages);
+			return 0;
+		}
+		(*nr)++;
+		pfn++;
+	} while (addr += PAGE_SIZE, addr != end);
+
+	if (pgmap)
+		put_dev_pagemap(pgmap);
+	return 1;
+}
+
+static int __gup_device_huge_pmd(pmd_t orig, pmd_t *pmdp, unsigned long addr,
+				 unsigned long end, unsigned int flags,
+				 struct page **pages, int *nr)
+{
+	unsigned long fault_pfn;
+	int nr_start = *nr;
+
+	fault_pfn = pmd_pfn(orig) + ((addr & ~PMD_MASK) >> PAGE_SHIFT);
+	if (!__gup_device_huge(fault_pfn, addr, end, flags, pages, nr))
+		return 0;
+
+	if (unlikely(pmd_val(orig) != pmd_val(*pmdp))) {
+		undo_dev_pagemap(nr, nr_start, flags, pages);
+		return 0;
+	}
+	return 1;
+}
+
+static int __gup_device_huge_pud(pud_t orig, pud_t *pudp, unsigned long addr,
+				 unsigned long end, unsigned int flags,
+				 struct page **pages, int *nr)
+{
+	unsigned long fault_pfn;
+	int nr_start = *nr;
+
+	fault_pfn = pud_pfn(orig) + ((addr & ~PUD_MASK) >> PAGE_SHIFT);
+	if (!__gup_device_huge(fault_pfn, addr, end, flags, pages, nr))
+		return 0;
+
+	if (unlikely(pud_val(orig) != pud_val(*pudp))) {
+		undo_dev_pagemap(nr, nr_start, flags, pages);
+		return 0;
+	}
+	return 1;
+}
+#else
+static int __gup_device_huge_pmd(pmd_t orig, pmd_t *pmdp, unsigned long addr,
+				 unsigned long end, unsigned int flags,
+				 struct page **pages, int *nr)
+{
+	BUILD_BUG();
+	return 0;
+}
+
+static int __gup_device_huge_pud(pud_t pud, pud_t *pudp, unsigned long addr,
+				 unsigned long end, unsigned int flags,
+				 struct page **pages, int *nr)
+{
+	BUILD_BUG();
+	return 0;
+}
+#endif
+
+static int record_subpages(struct page *page, unsigned long addr,
+			   unsigned long end, struct page **pages)
+{
+	int nr;
+
+	for (nr = 0; addr != end; addr += PAGE_SIZE)
+		pages[nr++] = page++;
+
+	return nr;
+}
+
+#ifdef CONFIG_ARCH_HAS_HUGEPD
+static unsigned long hugepte_addr_end(unsigned long addr, unsigned long end,
+				      unsigned long sz)
+{
+	unsigned long __boundary = (addr + sz) & ~(sz-1);
+	return (__boundary - 1 < end - 1) ? __boundary : end;
+}
+
+static int gup_hugepte(pte_t *ptep, unsigned long sz, unsigned long addr,
+		       unsigned long end, unsigned int flags,
+		       struct page **pages, int *nr)
+{
+	unsigned long pte_end;
+	struct page *head, *page;
+	pte_t pte;
+	int refs;
+
+	pte_end = (addr + sz) & ~(sz-1);
+	if (pte_end < end)
+		end = pte_end;
+
+	pte = huge_ptep_get(ptep);
+
+	if (!pte_access_permitted(pte, flags & FOLL_WRITE))
+		return 0;
+
+	/* hugepages are never "special" */
+	VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
+
+	head = pte_page(pte);
+	page = head + ((addr & (sz-1)) >> PAGE_SHIFT);
+	refs = record_subpages(page, addr, end, pages + *nr);
+
+	head = try_grab_compound_head(head, refs, flags);
+	if (!head)
+		return 0;
+
+	if (unlikely(pte_val(pte) != pte_val(*ptep))) {
+		put_compound_head(head, refs, flags);
+		return 0;
+	}
+
+	*nr += refs;
+	SetPageReferenced(head);
+	return 1;
+}
+
+static int gup_huge_pd(hugepd_t hugepd, unsigned long addr,
+		unsigned int pdshift, unsigned long end, unsigned int flags,
+		struct page **pages, int *nr)
+{
+	pte_t *ptep;
+	unsigned long sz = 1UL << hugepd_shift(hugepd);
+	unsigned long next;
+
+	ptep = hugepte_offset(hugepd, addr, pdshift);
+	do {
+		next = hugepte_addr_end(addr, end, sz);
+		if (!gup_hugepte(ptep, sz, addr, end, flags, pages, nr))
+			return 0;
+	} while (ptep++, addr = next, addr != end);
+
+	return 1;
+}
+#else
+static inline int gup_huge_pd(hugepd_t hugepd, unsigned long addr,
+		unsigned int pdshift, unsigned long end, unsigned int flags,
+		struct page **pages, int *nr)
+{
+	return 0;
+}
+#endif /* CONFIG_ARCH_HAS_HUGEPD */
+
+static int gup_huge_pmd(pmd_t orig, pmd_t *pmdp, unsigned long addr,
+			unsigned long end, unsigned int flags,
+			struct page **pages, int *nr)
+{
+	struct page *head, *page;
+	int refs;
+
+	if (!pmd_access_permitted(orig, flags & FOLL_WRITE))
+		return 0;
+
+	if (pmd_devmap(orig)) {
+		if (unlikely(flags & FOLL_LONGTERM))
+			return 0;
+		return __gup_device_huge_pmd(orig, pmdp, addr, end, flags,
+					     pages, nr);
+	}
+
+	page = pmd_page(orig) + ((addr & ~PMD_MASK) >> PAGE_SHIFT);
+	refs = record_subpages(page, addr, end, pages + *nr);
+
+	head = try_grab_compound_head(pmd_page(orig), refs, flags);
+	if (!head)
+		return 0;
+
+	if (unlikely(pmd_val(orig) != pmd_val(*pmdp))) {
+		put_compound_head(head, refs, flags);
+		return 0;
+	}
+
+	*nr += refs;
+	SetPageReferenced(head);
+	return 1;
+}
+
+static int gup_huge_pud(pud_t orig, pud_t *pudp, unsigned long addr,
+			unsigned long end, unsigned int flags,
+			struct page **pages, int *nr)
+{
+	struct page *head, *page;
+	int refs;
+
+	if (!pud_access_permitted(orig, flags & FOLL_WRITE))
+		return 0;
+
+	if (pud_devmap(orig)) {
+		if (unlikely(flags & FOLL_LONGTERM))
+			return 0;
+		return __gup_device_huge_pud(orig, pudp, addr, end, flags,
+					     pages, nr);
+	}
+
+	page = pud_page(orig) + ((addr & ~PUD_MASK) >> PAGE_SHIFT);
+	refs = record_subpages(page, addr, end, pages + *nr);
+
+	head = try_grab_compound_head(pud_page(orig), refs, flags);
+	if (!head)
+		return 0;
+
+	if (unlikely(pud_val(orig) != pud_val(*pudp))) {
+		put_compound_head(head, refs, flags);
+		return 0;
+	}
+
+	*nr += refs;
+	SetPageReferenced(head);
+	return 1;
+}
+
+static int gup_huge_pgd(pgd_t orig, pgd_t *pgdp, unsigned long addr,
+			unsigned long end, unsigned int flags,
+			struct page **pages, int *nr)
+{
+	int refs;
+	struct page *head, *page;
+
+	if (!pgd_access_permitted(orig, flags & FOLL_WRITE))
+		return 0;
+
+	BUILD_BUG_ON(pgd_devmap(orig));
+
+	page = pgd_page(orig) + ((addr & ~PGDIR_MASK) >> PAGE_SHIFT);
+	refs = record_subpages(page, addr, end, pages + *nr);
+
+	head = try_grab_compound_head(pgd_page(orig), refs, flags);
+	if (!head)
+		return 0;
+
+	if (unlikely(pgd_val(orig) != pgd_val(*pgdp))) {
+		put_compound_head(head, refs, flags);
+		return 0;
+	}
+
+	*nr += refs;
+	SetPageReferenced(head);
+	return 1;
+}
+
+static int gup_pmd_range(pud_t *pudp, pud_t pud, unsigned long addr, unsigned long end,
+		unsigned int flags, struct page **pages, int *nr)
+{
+	unsigned long next;
+	pmd_t *pmdp;
+
+	pmdp = pmd_offset_lockless(pudp, pud, addr);
+	do {
+		pmd_t pmd = READ_ONCE(*pmdp);
+
+		next = pmd_addr_end(addr, end);
+		if (!pmd_present(pmd))
+			return 0;
+
+		if (unlikely(pmd_trans_huge(pmd) || pmd_huge(pmd) ||
+			     pmd_devmap(pmd))) {
+			/*
+			 * NUMA hinting faults need to be handled in the GUP
+			 * slowpath for accounting purposes and so that they
+			 * can be serialised against THP migration.
+			 */
+			if (pmd_protnone(pmd))
+				return 0;
+
+			if (!gup_huge_pmd(pmd, pmdp, addr, next, flags,
+				pages, nr))
+				return 0;
+
+		} else if (unlikely(is_hugepd(__hugepd(pmd_val(pmd))))) {
+			/*
+			 * architecture have different format for hugetlbfs
+			 * pmd format and THP pmd format
+			 */
+			if (!gup_huge_pd(__hugepd(pmd_val(pmd)), addr,
+					 PMD_SHIFT, next, flags, pages, nr))
+				return 0;
+		} else if (!gup_pte_range(pmd, pmdp, addr, next, flags, pages, nr))
+			return 0;
+	} while (pmdp++, addr = next, addr != end);
+
+	return 1;
+}
+
+static int gup_pud_range(p4d_t *p4dp, p4d_t p4d, unsigned long addr, unsigned long end,
+			 unsigned int flags, struct page **pages, int *nr)
+{
+	unsigned long next;
+	pud_t *pudp;
+
+	pudp = pud_offset_lockless(p4dp, p4d, addr);
+	do {
+		pud_t pud = READ_ONCE(*pudp);
+
+		next = pud_addr_end(addr, end);
+		if (unlikely(!pud_present(pud)))
+			return 0;
+		if (unlikely(pud_huge(pud) || pud_devmap(pud))) {
+			if (!gup_huge_pud(pud, pudp, addr, next, flags,
+					  pages, nr))
+				return 0;
+		} else if (unlikely(is_hugepd(__hugepd(pud_val(pud))))) {
+			if (!gup_huge_pd(__hugepd(pud_val(pud)), addr,
+					 PUD_SHIFT, next, flags, pages, nr))
+				return 0;
+		} else if (!gup_pmd_range(pudp, pud, addr, next, flags, pages, nr))
+			return 0;
+	} while (pudp++, addr = next, addr != end);
+
+	return 1;
+}
+
+static int gup_p4d_range(pgd_t *pgdp, pgd_t pgd, unsigned long addr, unsigned long end,
+			 unsigned int flags, struct page **pages, int *nr)
+{
+	unsigned long next;
+	p4d_t *p4dp;
+
+	p4dp = p4d_offset_lockless(pgdp, pgd, addr);
+	do {
+		p4d_t p4d = READ_ONCE(*p4dp);
+
+		next = p4d_addr_end(addr, end);
+		if (p4d_none(p4d))
+			return 0;
+		BUILD_BUG_ON(p4d_huge(p4d));
+		if (unlikely(is_hugepd(__hugepd(p4d_val(p4d))))) {
+			if (!gup_huge_pd(__hugepd(p4d_val(p4d)), addr,
+					 P4D_SHIFT, next, flags, pages, nr))
+				return 0;
+		} else if (!gup_pud_range(p4dp, p4d, addr, next, flags, pages, nr))
+			return 0;
+	} while (p4dp++, addr = next, addr != end);
+
+	return 1;
+}
+
+static void gup_pgd_range(unsigned long addr, unsigned long end,
+		unsigned int flags, struct page **pages, int *nr)
+{
+	unsigned long next;
+	pgd_t *pgdp;
+
+	pgdp = pgd_offset(current->mm, addr);
+	do {
+		pgd_t pgd = READ_ONCE(*pgdp);
+
+		next = pgd_addr_end(addr, end);
+		if (pgd_none(pgd))
+			return;
+		if (unlikely(pgd_huge(pgd))) {
+			if (!gup_huge_pgd(pgd, pgdp, addr, next, flags,
+					  pages, nr))
+				return;
+		} else if (unlikely(is_hugepd(__hugepd(pgd_val(pgd))))) {
+			if (!gup_huge_pd(__hugepd(pgd_val(pgd)), addr,
+					 PGDIR_SHIFT, next, flags, pages, nr))
+				return;
+		} else if (!gup_p4d_range(pgdp, pgd, addr, next, flags, pages, nr))
+			return;
+	} while (pgdp++, addr = next, addr != end);
+}
+#else
+static inline void gup_pgd_range(unsigned long addr, unsigned long end,
+		unsigned int flags, struct page **pages, int *nr)
+{
+}
+#endif /* CONFIG_HAVE_FAST_GUP */
+
+#ifndef gup_fast_permitted
+/*
+ * Check if it's allowed to use get_user_pages_fast_only() for the range, or
+ * we need to fall back to the slow version:
+ */
+static bool gup_fast_permitted(unsigned long start, unsigned long end)
+{
+	return true;
+}
+#endif
+
+static int __gup_longterm_unlocked(unsigned long start, int nr_pages,
+				   unsigned int gup_flags, struct page **pages)
+{
+	int ret;
+
+	/*
+	 * FIXME: FOLL_LONGTERM does not work with
+	 * get_user_pages_unlocked() (see comments in that function)
+	 */
+	if (gup_flags & FOLL_LONGTERM) {
+		mmap_read_lock(current->mm);
+		ret = __gup_longterm_locked(current->mm,
+					    start, nr_pages,
+					    pages, NULL, gup_flags);
+		mmap_read_unlock(current->mm);
+	} else {
+		ret = get_user_pages_unlocked(start, nr_pages,
+					      pages, gup_flags);
+	}
+
+	return ret;
+}
+
+static unsigned long lockless_pages_from_mm(unsigned long start,
+					    unsigned long end,
+					    unsigned int gup_flags,
+					    struct page **pages)
+{
+	unsigned long flags;
+	int nr_pinned = 0;
+	unsigned seq;
+
+	if (!IS_ENABLED(CONFIG_HAVE_FAST_GUP) ||
+	    !gup_fast_permitted(start, end))
+		return 0;
+
+	if (gup_flags & FOLL_PIN) {
+		seq = raw_read_seqcount(&current->mm->write_protect_seq);
+		if (seq & 1)
+			return 0;
+	}
+
+	/*
+	 * Disable interrupts. The nested form is used, in order to allow full,
+	 * general purpose use of this routine.
+	 *
+	 * With interrupts disabled, we block page table pages from being freed
+	 * from under us. See struct mmu_table_batch comments in
+	 * include/asm-generic/tlb.h for more details.
+	 *
+	 * We do not adopt an rcu_read_lock() here as we also want to block IPIs
+	 * that come from THPs splitting.
+	 */
+	local_irq_save(flags);
+	gup_pgd_range(start, end, gup_flags, pages, &nr_pinned);
+	local_irq_restore(flags);
+
+	/*
+	 * When pinning pages for DMA there could be a concurrent write protect
+	 * from fork() via copy_page_range(), in this case always fail fast GUP.
+	 */
+	if (gup_flags & FOLL_PIN) {
+		if (read_seqcount_retry(&current->mm->write_protect_seq, seq)) {
+			unpin_user_pages(pages, nr_pinned);
+			return 0;
+		}
+	}
+	return nr_pinned;
+}
+
+static int internal_get_user_pages_fast(unsigned long start,
+					unsigned long nr_pages,
+					unsigned int gup_flags,
+					struct page **pages)
+{
+	unsigned long len, end;
+	unsigned long nr_pinned;
+	int ret;
+
+	if (WARN_ON_ONCE(gup_flags & ~(FOLL_WRITE | FOLL_LONGTERM |
+				       FOLL_FORCE | FOLL_PIN | FOLL_GET |
+				       FOLL_FAST_ONLY)))
+		return -EINVAL;
+
+	if (gup_flags & FOLL_PIN)
+		atomic_set(&current->mm->has_pinned, 1);
+
+	if (!(gup_flags & FOLL_FAST_ONLY))
+		might_lock_read(&current->mm->mmap_lock);
+
+	start = untagged_addr(start) & PAGE_MASK;
+	len = nr_pages << PAGE_SHIFT;
+	if (check_add_overflow(start, len, &end))
+		return 0;
+	if (unlikely(!access_ok((void __user *)start, len)))
+		return -EFAULT;
+
+	nr_pinned = lockless_pages_from_mm(start, end, gup_flags, pages);
+	if (nr_pinned == nr_pages || gup_flags & FOLL_FAST_ONLY)
+		return nr_pinned;
+
+	/* Slow path: try to get the remaining pages with get_user_pages */
+	start += nr_pinned << PAGE_SHIFT;
+	pages += nr_pinned;
+	ret = __gup_longterm_unlocked(start, nr_pages - nr_pinned, gup_flags,
+				      pages);
+	if (ret < 0) {
+		/*
+		 * The caller has to unpin the pages we already pinned so
+		 * returning -errno is not an option
+		 */
+		if (nr_pinned)
+			return nr_pinned;
+		return ret;
+	}
+	return ret + nr_pinned;
+}
+
+/**
+ * get_user_pages_fast_only() - pin user pages in memory
+ * @start:      starting user address
+ * @nr_pages:   number of pages from start to pin
+ * @gup_flags:  flags modifying pin behaviour
+ * @pages:      array that receives pointers to the pages pinned.
+ *              Should be at least nr_pages long.
+ *
+ * Like get_user_pages_fast() except it's IRQ-safe in that it won't fall back to
+ * the regular GUP.
+ * Note a difference with get_user_pages_fast: this always returns the
+ * number of pages pinned, 0 if no pages were pinned.
+ *
+ * If the architecture does not support this function, simply return with no
+ * pages pinned.
+ *
+ * Careful, careful! COW breaking can go either way, so a non-write
+ * access can get ambiguous page results. If you call this function without
+ * 'write' set, you'd better be sure that you're ok with that ambiguity.
+ */
+int get_user_pages_fast_only(unsigned long start, int nr_pages,
+			     unsigned int gup_flags, struct page **pages)
+{
+	int nr_pinned;
+	/*
+	 * Internally (within mm/gup.c), gup fast variants must set FOLL_GET,
+	 * because gup fast is always a "pin with a +1 page refcount" request.
+	 *
+	 * FOLL_FAST_ONLY is required in order to match the API description of
+	 * this routine: no fall back to regular ("slow") GUP.
+	 */
+	gup_flags |= FOLL_GET | FOLL_FAST_ONLY;
+
+	nr_pinned = internal_get_user_pages_fast(start, nr_pages, gup_flags,
+						 pages);
+
+	/*
+	 * As specified in the API description above, this routine is not
+	 * allowed to return negative values. However, the common core
+	 * routine internal_get_user_pages_fast() *can* return -errno.
+	 * Therefore, correct for that here:
+	 */
+	if (nr_pinned < 0)
+		nr_pinned = 0;
+
+	return nr_pinned;
+}
+EXPORT_SYMBOL_GPL(get_user_pages_fast_only);
+
+/**
+ * get_user_pages_fast() - pin user pages in memory
+ * @start:      starting user address
+ * @nr_pages:   number of pages from start to pin
+ * @gup_flags:  flags modifying pin behaviour
+ * @pages:      array that receives pointers to the pages pinned.
+ *              Should be at least nr_pages long.
+ *
+ * Attempt to pin user pages in memory without taking mm->mmap_lock.
+ * If not successful, it will fall back to taking the lock and
+ * calling get_user_pages().
+ *
+ * Returns number of pages pinned. This may be fewer than the number requested.
+ * If nr_pages is 0 or negative, returns 0. If no pages were pinned, returns
+ * -errno.
+ */
+int get_user_pages_fast(unsigned long start, int nr_pages,
+			unsigned int gup_flags, struct page **pages)
+{
+	if (!is_valid_gup_flags(gup_flags))
+		return -EINVAL;
+
+	/*
+	 * The caller may or may not have explicitly set FOLL_GET; either way is
+	 * OK. However, internally (within mm/gup.c), gup fast variants must set
+	 * FOLL_GET, because gup fast is always a "pin with a +1 page refcount"
+	 * request.
+	 */
+	gup_flags |= FOLL_GET;
+	return internal_get_user_pages_fast(start, nr_pages, gup_flags, pages);
+}
+EXPORT_SYMBOL_GPL(get_user_pages_fast);
+
+/**
+ * pin_user_pages_fast() - pin user pages in memory without taking locks
+ *
+ * @start:      starting user address
+ * @nr_pages:   number of pages from start to pin
+ * @gup_flags:  flags modifying pin behaviour
+ * @pages:      array that receives pointers to the pages pinned.
+ *              Should be at least nr_pages long.
+ *
+ * Nearly the same as get_user_pages_fast(), except that FOLL_PIN is set. See
+ * get_user_pages_fast() for documentation on the function arguments, because
+ * the arguments here are identical.
+ *
+ * FOLL_PIN means that the pages must be released via unpin_user_page(). Please
+ * see Documentation/core-api/pin_user_pages.rst for further details.
+ */
+int pin_user_pages_fast(unsigned long start, int nr_pages,
+			unsigned int gup_flags, struct page **pages)
+{
+	/* FOLL_GET and FOLL_PIN are mutually exclusive. */
+	if (WARN_ON_ONCE(gup_flags & FOLL_GET))
+		return -EINVAL;
+
+	gup_flags |= FOLL_PIN;
+	return internal_get_user_pages_fast(start, nr_pages, gup_flags, pages);
+}
+EXPORT_SYMBOL_GPL(pin_user_pages_fast);
+
+/*
+ * This is the FOLL_PIN equivalent of get_user_pages_fast_only(). Behavior
+ * is the same, except that this one sets FOLL_PIN instead of FOLL_GET.
+ *
+ * The API rules are the same, too: no negative values may be returned.
+ */
+int pin_user_pages_fast_only(unsigned long start, int nr_pages,
+			     unsigned int gup_flags, struct page **pages)
+{
+	int nr_pinned;
+
+	/*
+	 * FOLL_GET and FOLL_PIN are mutually exclusive. Note that the API
+	 * rules require returning 0, rather than -errno:
+	 */
+	if (WARN_ON_ONCE(gup_flags & FOLL_GET))
+		return 0;
+	/*
+	 * FOLL_FAST_ONLY is required in order to match the API description of
+	 * this routine: no fall back to regular ("slow") GUP.
+	 */
+	gup_flags |= (FOLL_PIN | FOLL_FAST_ONLY);
+	nr_pinned = internal_get_user_pages_fast(start, nr_pages, gup_flags,
+						 pages);
+	/*
+	 * This routine is not allowed to return negative values. However,
+	 * internal_get_user_pages_fast() *can* return -errno. Therefore,
+	 * correct for that here:
+	 */
+	if (nr_pinned < 0)
+		nr_pinned = 0;
+
+	return nr_pinned;
+}
+EXPORT_SYMBOL_GPL(pin_user_pages_fast_only);
+
+/**
+ * pin_user_pages_remote() - pin pages of a remote process
+ *
+ * @mm:		mm_struct of target mm
+ * @start:	starting user address
+ * @nr_pages:	number of pages from start to pin
+ * @gup_flags:	flags modifying lookup behaviour
+ * @pages:	array that receives pointers to the pages pinned.
+ *		Should be at least nr_pages long. Or NULL, if caller
+ *		only intends to ensure the pages are faulted in.
+ * @vmas:	array of pointers to vmas corresponding to each page.
+ *		Or NULL if the caller does not require them.
+ * @locked:	pointer to lock flag indicating whether lock is held and
+ *		subsequently whether VM_FAULT_RETRY functionality can be
+ *		utilised. Lock must initially be held.
+ *
+ * Nearly the same as get_user_pages_remote(), except that FOLL_PIN is set. See
+ * get_user_pages_remote() for documentation on the function arguments, because
+ * the arguments here are identical.
+ *
+ * FOLL_PIN means that the pages must be released via unpin_user_page(). Please
+ * see Documentation/core-api/pin_user_pages.rst for details.
+ */
+long pin_user_pages_remote(struct mm_struct *mm,
+			   unsigned long start, unsigned long nr_pages,
+			   unsigned int gup_flags, struct page **pages,
+			   struct vm_area_struct **vmas, int *locked)
+{
+	/* FOLL_GET and FOLL_PIN are mutually exclusive. */
+	if (WARN_ON_ONCE(gup_flags & FOLL_GET))
+		return -EINVAL;
+
+	gup_flags |= FOLL_PIN;
+	return __get_user_pages_remote(mm, start, nr_pages, gup_flags,
+				       pages, vmas, locked);
+}
+EXPORT_SYMBOL(pin_user_pages_remote);
+
+/**
+ * pin_user_pages() - pin user pages in memory for use by other devices
+ *
+ * @start:	starting user address
+ * @nr_pages:	number of pages from start to pin
+ * @gup_flags:	flags modifying lookup behaviour
+ * @pages:	array that receives pointers to the pages pinned.
+ *		Should be at least nr_pages long. Or NULL, if caller
+ *		only intends to ensure the pages are faulted in.
+ * @vmas:	array of pointers to vmas corresponding to each page.
+ *		Or NULL if the caller does not require them.
+ *
+ * Nearly the same as get_user_pages(), except that FOLL_TOUCH is not set, and
+ * FOLL_PIN is set.
+ *
+ * FOLL_PIN means that the pages must be released via unpin_user_page(). Please
+ * see Documentation/core-api/pin_user_pages.rst for details.
+ */
+long pin_user_pages(unsigned long start, unsigned long nr_pages,
+		    unsigned int gup_flags, struct page **pages,
+		    struct vm_area_struct **vmas)
+{
+	/* FOLL_GET and FOLL_PIN are mutually exclusive. */
+	if (WARN_ON_ONCE(gup_flags & FOLL_GET))
+		return -EINVAL;
+
+	gup_flags |= FOLL_PIN;
+	return __gup_longterm_locked(current->mm, start, nr_pages,
+				     pages, vmas, gup_flags);
+}
+EXPORT_SYMBOL(pin_user_pages);
+
+/*
+ * pin_user_pages_unlocked() is the FOLL_PIN variant of
+ * get_user_pages_unlocked(). Behavior is the same, except that this one sets
+ * FOLL_PIN and rejects FOLL_GET.
+ */
+long pin_user_pages_unlocked(unsigned long start, unsigned long nr_pages,
+			     struct page **pages, unsigned int gup_flags)
+{
+	/* FOLL_GET and FOLL_PIN are mutually exclusive. */
+	if (WARN_ON_ONCE(gup_flags & FOLL_GET))
+		return -EINVAL;
+
+	gup_flags |= FOLL_PIN;
+	return get_user_pages_unlocked(start, nr_pages, pages, gup_flags);
+}
+EXPORT_SYMBOL(pin_user_pages_unlocked);
+
+/*
+ * pin_user_pages_locked() is the FOLL_PIN variant of get_user_pages_locked().
+ * Behavior is the same, except that this one sets FOLL_PIN and rejects
+ * FOLL_GET.
+ */
+long pin_user_pages_locked(unsigned long start, unsigned long nr_pages,
+			   unsigned int gup_flags, struct page **pages,
+			   int *locked)
+{
+	/*
+	 * FIXME: Current FOLL_LONGTERM behavior is incompatible with
+	 * FAULT_FLAG_ALLOW_RETRY because of the FS DAX check requirement on
+	 * vmas.  As there are no users of this flag in this call we simply
+	 * disallow this option for now.
+	 */
+	if (WARN_ON_ONCE(gup_flags & FOLL_LONGTERM))
+		return -EINVAL;
+
+	/* FOLL_GET and FOLL_PIN are mutually exclusive. */
+	if (WARN_ON_ONCE(gup_flags & FOLL_GET))
+		return -EINVAL;
+
+	gup_flags |= FOLL_PIN;
+	return __get_user_pages_locked(current->mm, start, nr_pages,
+				       pages, NULL, locked,
+				       gup_flags | FOLL_TOUCH);
+}
+EXPORT_SYMBOL(pin_user_pages_locked);