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+============================
+Transparent Hugepage Support
+============================
+
+This document describes design principles for Transparent Hugepage (THP)
+support and its interaction with other parts of the memory management
+system.
+
+Design principles
+=================
+
+- "graceful fallback": mm components which don't have transparent hugepage
+  knowledge fall back to breaking huge pmd mapping into table of ptes and,
+  if necessary, split a transparent hugepage. Therefore these components
+  can continue working on the regular pages or regular pte mappings.
+
+- if a hugepage allocation fails because of memory fragmentation,
+  regular pages should be gracefully allocated instead and mixed in
+  the same vma without any failure or significant delay and without
+  userland noticing
+
+- if some task quits and more hugepages become available (either
+  immediately in the buddy or through the VM), guest physical memory
+  backed by regular pages should be relocated on hugepages
+  automatically (with khugepaged)
+
+- it doesn't require memory reservation and in turn it uses hugepages
+  whenever possible (the only possible reservation here is kernelcore=
+  to avoid unmovable pages to fragment all the memory but such a tweak
+  is not specific to transparent hugepage support and it's a generic
+  feature that applies to all dynamic high order allocations in the
+  kernel)
+
+get_user_pages and follow_page
+==============================
+
+get_user_pages and follow_page if run on a hugepage, will return the
+head or tail pages as usual (exactly as they would do on
+hugetlbfs). Most GUP users will only care about the actual physical
+address of the page and its temporary pinning to release after the I/O
+is complete, so they won't ever notice the fact the page is huge. But
+if any driver is going to mangle over the page structure of the tail
+page (like for checking page->mapping or other bits that are relevant
+for the head page and not the tail page), it should be updated to jump
+to check head page instead. Taking a reference on any head/tail page would
+prevent the page from being split by anyone.
+
+.. note::
+   these aren't new constraints to the GUP API, and they match the
+   same constraints that apply to hugetlbfs too, so any driver capable
+   of handling GUP on hugetlbfs will also work fine on transparent
+   hugepage backed mappings.
+
+Graceful fallback
+=================
+
+Code walking pagetables but unaware about huge pmds can simply call
+split_huge_pmd(vma, pmd, addr) where the pmd is the one returned by
+pmd_offset. It's trivial to make the code transparent hugepage aware
+by just grepping for "pmd_offset" and adding split_huge_pmd where
+missing after pmd_offset returns the pmd. Thanks to the graceful
+fallback design, with a one liner change, you can avoid to write
+hundreds if not thousands of lines of complex code to make your code
+hugepage aware.
+
+If you're not walking pagetables but you run into a physical hugepage
+that you can't handle natively in your code, you can split it by
+calling split_huge_page(page). This is what the Linux VM does before
+it tries to swapout the hugepage for example. split_huge_page() can fail
+if the page is pinned and you must handle this correctly.
+
+Example to make mremap.c transparent hugepage aware with a one liner
+change::
+
+	diff --git a/mm/mremap.c b/mm/mremap.c
+	--- a/mm/mremap.c
+	+++ b/mm/mremap.c
+	@@ -41,6 +41,7 @@ static pmd_t *get_old_pmd(struct mm_stru
+			return NULL;
+
+		pmd = pmd_offset(pud, addr);
+	+	split_huge_pmd(vma, pmd, addr);
+		if (pmd_none_or_clear_bad(pmd))
+			return NULL;
+
+Locking in hugepage aware code
+==============================
+
+We want as much code as possible hugepage aware, as calling
+split_huge_page() or split_huge_pmd() has a cost.
+
+To make pagetable walks huge pmd aware, all you need to do is to call
+pmd_trans_huge() on the pmd returned by pmd_offset. You must hold the
+mmap_lock in read (or write) mode to be sure a huge pmd cannot be
+created from under you by khugepaged (khugepaged collapse_huge_page
+takes the mmap_lock in write mode in addition to the anon_vma lock). If
+pmd_trans_huge returns false, you just fallback in the old code
+paths. If instead pmd_trans_huge returns true, you have to take the
+page table lock (pmd_lock()) and re-run pmd_trans_huge. Taking the
+page table lock will prevent the huge pmd being converted into a
+regular pmd from under you (split_huge_pmd can run in parallel to the
+pagetable walk). If the second pmd_trans_huge returns false, you
+should just drop the page table lock and fallback to the old code as
+before. Otherwise, you can proceed to process the huge pmd and the
+hugepage natively. Once finished, you can drop the page table lock.
+
+Refcounts and transparent huge pages
+====================================
+
+Refcounting on THP is mostly consistent with refcounting on other compound
+pages:
+
+  - get_page()/put_page() and GUP operate on the folio->_refcount.
+
+  - ->_refcount in tail pages is always zero: get_page_unless_zero() never
+    succeeds on tail pages.
+
+  - map/unmap of a PMD entry for the whole THP increment/decrement
+    folio->_entire_mapcount and also increment/decrement
+    folio->_nr_pages_mapped by COMPOUND_MAPPED when _entire_mapcount
+    goes from -1 to 0 or 0 to -1.
+
+  - map/unmap of individual pages with PTE entry increment/decrement
+    page->_mapcount and also increment/decrement folio->_nr_pages_mapped
+    when page->_mapcount goes from -1 to 0 or 0 to -1 as this counts
+    the number of pages mapped by PTE.
+
+split_huge_page internally has to distribute the refcounts in the head
+page to the tail pages before clearing all PG_head/tail bits from the page
+structures. It can be done easily for refcounts taken by page table
+entries, but we don't have enough information on how to distribute any
+additional pins (i.e. from get_user_pages). split_huge_page() fails any
+requests to split pinned huge pages: it expects page count to be equal to
+the sum of mapcount of all sub-pages plus one (split_huge_page caller must
+have a reference to the head page).
+
+split_huge_page uses migration entries to stabilize page->_refcount and
+page->_mapcount of anonymous pages. File pages just get unmapped.
+
+We are safe against physical memory scanners too: the only legitimate way
+a scanner can get a reference to a page is get_page_unless_zero().
+
+All tail pages have zero ->_refcount until atomic_add(). This prevents the
+scanner from getting a reference to the tail page up to that point. After the
+atomic_add() we don't care about the ->_refcount value. We already know how
+many references should be uncharged from the head page.
+
+For head page get_page_unless_zero() will succeed and we don't mind. It's
+clear where references should go after split: it will stay on the head page.
+
+Note that split_huge_pmd() doesn't have any limitations on refcounting:
+pmd can be split at any point and never fails.
+
+Partial unmap and deferred_split_folio()
+========================================
+
+Unmapping part of THP (with munmap() or other way) is not going to free
+memory immediately. Instead, we detect that a subpage of THP is not in use
+in page_remove_rmap() and queue the THP for splitting if memory pressure
+comes. Splitting will free up unused subpages.
+
+Splitting the page right away is not an option due to locking context in
+the place where we can detect partial unmap. It also might be
+counterproductive since in many cases partial unmap happens during exit(2) if
+a THP crosses a VMA boundary.
+
+The function deferred_split_folio() is used to queue a folio for splitting.
+The splitting itself will happen when we get memory pressure via shrinker
+interface.