1 files changed, 497 insertions, 0 deletions
diff --git a/mm/frontswap.c b/mm/frontswap.c
new file mode 100644
index 000000000..2183a56c7
--- /dev/null
+++ b/mm/frontswap.c
@@ -0,0 +1,497 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Frontswap frontend
+ *
+ * This code provides the generic "frontend" layer to call a matching
+ * "backend" driver implementation of frontswap.  See
+ * Documentation/vm/frontswap.rst for more information.
+ *
+ * Copyright (C) 2009-2012 Oracle Corp.  All rights reserved.
+ * Author: Dan Magenheimer
+ */
+
+#include <linux/mman.h>
+#include <linux/swap.h>
+#include <linux/swapops.h>
+#include <linux/security.h>
+#include <linux/module.h>
+#include <linux/debugfs.h>
+#include <linux/frontswap.h>
+#include <linux/swapfile.h>
+
+DEFINE_STATIC_KEY_FALSE(frontswap_enabled_key);
+
+/*
+ * frontswap_ops are added by frontswap_register_ops, and provide the
+ * frontswap "backend" implementation functions.  Multiple implementations
+ * may be registered, but implementations can never deregister.  This
+ * is a simple singly-linked list of all registered implementations.
+ */
+static struct frontswap_ops *frontswap_ops __read_mostly;
+
+#define for_each_frontswap_ops(ops)		\
+	for ((ops) = frontswap_ops; (ops); (ops) = (ops)->next)
+
+/*
+ * If enabled, frontswap_store will return failure even on success.  As
+ * a result, the swap subsystem will always write the page to swap, in
+ * effect converting frontswap into a writethrough cache.  In this mode,
+ * there is no direct reduction in swap writes, but a frontswap backend
+ * can unilaterally "reclaim" any pages in use with no data loss, thus
+ * providing increases control over maximum memory usage due to frontswap.
+ */
+static bool frontswap_writethrough_enabled __read_mostly;
+
+/*
+ * If enabled, the underlying tmem implementation is capable of doing
+ * exclusive gets, so frontswap_load, on a successful tmem_get must
+ * mark the page as no longer in frontswap AND mark it dirty.
+ */
+static bool frontswap_tmem_exclusive_gets_enabled __read_mostly;
+
+#ifdef CONFIG_DEBUG_FS
+/*
+ * Counters available via /sys/kernel/debug/frontswap (if debugfs is
+ * properly configured).  These are for information only so are not protected
+ * against increment races.
+ */
+static u64 frontswap_loads;
+static u64 frontswap_succ_stores;
+static u64 frontswap_failed_stores;
+static u64 frontswap_invalidates;
+
+static inline void inc_frontswap_loads(void) {
+	data_race(frontswap_loads++);
+}
+static inline void inc_frontswap_succ_stores(void) {
+	data_race(frontswap_succ_stores++);
+}
+static inline void inc_frontswap_failed_stores(void) {
+	data_race(frontswap_failed_stores++);
+}
+static inline void inc_frontswap_invalidates(void) {
+	data_race(frontswap_invalidates++);
+}
+#else
+static inline void inc_frontswap_loads(void) { }
+static inline void inc_frontswap_succ_stores(void) { }
+static inline void inc_frontswap_failed_stores(void) { }
+static inline void inc_frontswap_invalidates(void) { }
+#endif
+
+/*
+ * Due to the asynchronous nature of the backends loading potentially
+ * _after_ the swap system has been activated, we have chokepoints
+ * on all frontswap functions to not call the backend until the backend
+ * has registered.
+ *
+ * This would not guards us against the user deciding to call swapoff right as
+ * we are calling the backend to initialize (so swapon is in action).
+ * Fortunately for us, the swapon_mutex has been taken by the callee so we are
+ * OK. The other scenario where calls to frontswap_store (called via
+ * swap_writepage) is racing with frontswap_invalidate_area (called via
+ * swapoff) is again guarded by the swap subsystem.
+ *
+ * While no backend is registered all calls to frontswap_[store|load|
+ * invalidate_area|invalidate_page] are ignored or fail.
+ *
+ * The time between the backend being registered and the swap file system
+ * calling the backend (via the frontswap_* functions) is indeterminate as
+ * frontswap_ops is not atomic_t (or a value guarded by a spinlock).
+ * That is OK as we are comfortable missing some of these calls to the newly
+ * registered backend.
+ *
+ * Obviously the opposite (unloading the backend) must be done after all
+ * the frontswap_[store|load|invalidate_area|invalidate_page] start
+ * ignoring or failing the requests.  However, there is currently no way
+ * to unload a backend once it is registered.
+ */
+
+/*
+ * Register operations for frontswap
+ */
+void frontswap_register_ops(struct frontswap_ops *ops)
+{
+	DECLARE_BITMAP(a, MAX_SWAPFILES);
+	DECLARE_BITMAP(b, MAX_SWAPFILES);
+	struct swap_info_struct *si;
+	unsigned int i;
+
+	bitmap_zero(a, MAX_SWAPFILES);
+	bitmap_zero(b, MAX_SWAPFILES);
+
+	spin_lock(&swap_lock);
+	plist_for_each_entry(si, &swap_active_head, list) {
+		if (!WARN_ON(!si->frontswap_map))
+			set_bit(si->type, a);
+	}
+	spin_unlock(&swap_lock);
+
+	/* the new ops needs to know the currently active swap devices */
+	for_each_set_bit(i, a, MAX_SWAPFILES)
+		ops->init(i);
+
+	/*
+	 * Setting frontswap_ops must happen after the ops->init() calls
+	 * above; cmpxchg implies smp_mb() which will ensure the init is
+	 * complete at this point.
+	 */
+	do {
+		ops->next = frontswap_ops;
+	} while (cmpxchg(&frontswap_ops, ops->next, ops) != ops->next);
+
+	static_branch_inc(&frontswap_enabled_key);
+
+	spin_lock(&swap_lock);
+	plist_for_each_entry(si, &swap_active_head, list) {
+		if (si->frontswap_map)
+			set_bit(si->type, b);
+	}
+	spin_unlock(&swap_lock);
+
+	/*
+	 * On the very unlikely chance that a swap device was added or
+	 * removed between setting the "a" list bits and the ops init
+	 * calls, we re-check and do init or invalidate for any changed
+	 * bits.
+	 */
+	if (unlikely(!bitmap_equal(a, b, MAX_SWAPFILES))) {
+		for (i = 0; i < MAX_SWAPFILES; i++) {
+			if (!test_bit(i, a) && test_bit(i, b))
+				ops->init(i);
+			else if (test_bit(i, a) && !test_bit(i, b))
+				ops->invalidate_area(i);
+		}
+	}
+}
+EXPORT_SYMBOL(frontswap_register_ops);
+
+/*
+ * Enable/disable frontswap writethrough (see above).
+ */
+void frontswap_writethrough(bool enable)
+{
+	frontswap_writethrough_enabled = enable;
+}
+EXPORT_SYMBOL(frontswap_writethrough);
+
+/*
+ * Enable/disable frontswap exclusive gets (see above).
+ */
+void frontswap_tmem_exclusive_gets(bool enable)
+{
+	frontswap_tmem_exclusive_gets_enabled = enable;
+}
+EXPORT_SYMBOL(frontswap_tmem_exclusive_gets);
+
+/*
+ * Called when a swap device is swapon'd.
+ */
+void __frontswap_init(unsigned type, unsigned long *map)
+{
+	struct swap_info_struct *sis = swap_info[type];
+	struct frontswap_ops *ops;
+
+	VM_BUG_ON(sis == NULL);
+
+	/*
+	 * p->frontswap is a bitmap that we MUST have to figure out which page
+	 * has gone in frontswap. Without it there is no point of continuing.
+	 */
+	if (WARN_ON(!map))
+		return;
+	/*
+	 * Irregardless of whether the frontswap backend has been loaded
+	 * before this function or it will be later, we _MUST_ have the
+	 * p->frontswap set to something valid to work properly.
+	 */
+	frontswap_map_set(sis, map);
+
+	for_each_frontswap_ops(ops)
+		ops->init(type);
+}
+EXPORT_SYMBOL(__frontswap_init);
+
+bool __frontswap_test(struct swap_info_struct *sis,
+				pgoff_t offset)
+{
+	if (sis->frontswap_map)
+		return test_bit(offset, sis->frontswap_map);
+	return false;
+}
+EXPORT_SYMBOL(__frontswap_test);
+
+static inline void __frontswap_set(struct swap_info_struct *sis,
+				   pgoff_t offset)
+{
+	set_bit(offset, sis->frontswap_map);
+	atomic_inc(&sis->frontswap_pages);
+}
+
+static inline void __frontswap_clear(struct swap_info_struct *sis,
+				     pgoff_t offset)
+{
+	clear_bit(offset, sis->frontswap_map);
+	atomic_dec(&sis->frontswap_pages);
+}
+
+/*
+ * "Store" data from a page to frontswap and associate it with the page's
+ * swaptype and offset.  Page must be locked and in the swap cache.
+ * If frontswap already contains a page with matching swaptype and
+ * offset, the frontswap implementation may either overwrite the data and
+ * return success or invalidate the page from frontswap and return failure.
+ */
+int __frontswap_store(struct page *page)
+{
+	int ret = -1;
+	swp_entry_t entry = { .val = page_private(page), };
+	int type = swp_type(entry);
+	struct swap_info_struct *sis = swap_info[type];
+	pgoff_t offset = swp_offset(entry);
+	struct frontswap_ops *ops;
+
+	VM_BUG_ON(!frontswap_ops);
+	VM_BUG_ON(!PageLocked(page));
+	VM_BUG_ON(sis == NULL);
+
+	/*
+	 * If a dup, we must remove the old page first; we can't leave the
+	 * old page no matter if the store of the new page succeeds or fails,
+	 * and we can't rely on the new page replacing the old page as we may
+	 * not store to the same implementation that contains the old page.
+	 */
+	if (__frontswap_test(sis, offset)) {
+		__frontswap_clear(sis, offset);
+		for_each_frontswap_ops(ops)
+			ops->invalidate_page(type, offset);
+	}
+
+	/* Try to store in each implementation, until one succeeds. */
+	for_each_frontswap_ops(ops) {
+		ret = ops->store(type, offset, page);
+		if (!ret) /* successful store */
+			break;
+	}
+	if (ret == 0) {
+		__frontswap_set(sis, offset);
+		inc_frontswap_succ_stores();
+	} else {
+		inc_frontswap_failed_stores();
+	}
+	if (frontswap_writethrough_enabled)
+		/* report failure so swap also writes to swap device */
+		ret = -1;
+	return ret;
+}
+EXPORT_SYMBOL(__frontswap_store);
+
+/*
+ * "Get" data from frontswap associated with swaptype and offset that were
+ * specified when the data was put to frontswap and use it to fill the
+ * specified page with data. Page must be locked and in the swap cache.
+ */
+int __frontswap_load(struct page *page)
+{
+	int ret = -1;
+	swp_entry_t entry = { .val = page_private(page), };
+	int type = swp_type(entry);
+	struct swap_info_struct *sis = swap_info[type];
+	pgoff_t offset = swp_offset(entry);
+	struct frontswap_ops *ops;
+
+	VM_BUG_ON(!frontswap_ops);
+	VM_BUG_ON(!PageLocked(page));
+	VM_BUG_ON(sis == NULL);
+
+	if (!__frontswap_test(sis, offset))
+		return -1;
+
+	/* Try loading from each implementation, until one succeeds. */
+	for_each_frontswap_ops(ops) {
+		ret = ops->load(type, offset, page);
+		if (!ret) /* successful load */
+			break;
+	}
+	if (ret == 0) {
+		inc_frontswap_loads();
+		if (frontswap_tmem_exclusive_gets_enabled) {
+			SetPageDirty(page);
+			__frontswap_clear(sis, offset);
+		}
+	}
+	return ret;
+}
+EXPORT_SYMBOL(__frontswap_load);
+
+/*
+ * Invalidate any data from frontswap associated with the specified swaptype
+ * and offset so that a subsequent "get" will fail.
+ */
+void __frontswap_invalidate_page(unsigned type, pgoff_t offset)
+{
+	struct swap_info_struct *sis = swap_info[type];
+	struct frontswap_ops *ops;
+
+	VM_BUG_ON(!frontswap_ops);
+	VM_BUG_ON(sis == NULL);
+
+	if (!__frontswap_test(sis, offset))
+		return;
+
+	for_each_frontswap_ops(ops)
+		ops->invalidate_page(type, offset);
+	__frontswap_clear(sis, offset);
+	inc_frontswap_invalidates();
+}
+EXPORT_SYMBOL(__frontswap_invalidate_page);
+
+/*
+ * Invalidate all data from frontswap associated with all offsets for the
+ * specified swaptype.
+ */
+void __frontswap_invalidate_area(unsigned type)
+{
+	struct swap_info_struct *sis = swap_info[type];
+	struct frontswap_ops *ops;
+
+	VM_BUG_ON(!frontswap_ops);
+	VM_BUG_ON(sis == NULL);
+
+	if (sis->frontswap_map == NULL)
+		return;
+
+	for_each_frontswap_ops(ops)
+		ops->invalidate_area(type);
+	atomic_set(&sis->frontswap_pages, 0);
+	bitmap_zero(sis->frontswap_map, sis->max);
+}
+EXPORT_SYMBOL(__frontswap_invalidate_area);
+
+static unsigned long __frontswap_curr_pages(void)
+{
+	unsigned long totalpages = 0;
+	struct swap_info_struct *si = NULL;
+
+	assert_spin_locked(&swap_lock);
+	plist_for_each_entry(si, &swap_active_head, list)
+		totalpages += atomic_read(&si->frontswap_pages);
+	return totalpages;
+}
+
+static int __frontswap_unuse_pages(unsigned long total, unsigned long *unused,
+					int *swapid)
+{
+	int ret = -EINVAL;
+	struct swap_info_struct *si = NULL;
+	int si_frontswap_pages;
+	unsigned long total_pages_to_unuse = total;
+	unsigned long pages = 0, pages_to_unuse = 0;
+
+	assert_spin_locked(&swap_lock);
+	plist_for_each_entry(si, &swap_active_head, list) {
+		si_frontswap_pages = atomic_read(&si->frontswap_pages);
+		if (total_pages_to_unuse < si_frontswap_pages) {
+			pages = pages_to_unuse = total_pages_to_unuse;
+		} else {
+			pages = si_frontswap_pages;
+			pages_to_unuse = 0; /* unuse all */
+		}
+		/* ensure there is enough RAM to fetch pages from frontswap */
+		if (security_vm_enough_memory_mm(current->mm, pages)) {
+			ret = -ENOMEM;
+			continue;
+		}
+		vm_unacct_memory(pages);
+		*unused = pages_to_unuse;
+		*swapid = si->type;
+		ret = 0;
+		break;
+	}
+
+	return ret;
+}
+
+/*
+ * Used to check if it's necessary and feasible to unuse pages.
+ * Return 1 when nothing to do, 0 when need to shrink pages,
+ * error code when there is an error.
+ */
+static int __frontswap_shrink(unsigned long target_pages,
+				unsigned long *pages_to_unuse,
+				int *type)
+{
+	unsigned long total_pages = 0, total_pages_to_unuse;
+
+	assert_spin_locked(&swap_lock);
+
+	total_pages = __frontswap_curr_pages();
+	if (total_pages <= target_pages) {
+		/* Nothing to do */
+		*pages_to_unuse = 0;
+		return 1;
+	}
+	total_pages_to_unuse = total_pages - target_pages;
+	return __frontswap_unuse_pages(total_pages_to_unuse, pages_to_unuse, type);
+}
+
+/*
+ * Frontswap, like a true swap device, may unnecessarily retain pages
+ * under certain circumstances; "shrink" frontswap is essentially a
+ * "partial swapoff" and works by calling try_to_unuse to attempt to
+ * unuse enough frontswap pages to attempt to -- subject to memory
+ * constraints -- reduce the number of pages in frontswap to the
+ * number given in the parameter target_pages.
+ */
+void frontswap_shrink(unsigned long target_pages)
+{
+	unsigned long pages_to_unuse = 0;
+	int type, ret;
+
+	/*
+	 * we don't want to hold swap_lock while doing a very
+	 * lengthy try_to_unuse, but swap_list may change
+	 * so restart scan from swap_active_head each time
+	 */
+	spin_lock(&swap_lock);
+	ret = __frontswap_shrink(target_pages, &pages_to_unuse, &type);
+	spin_unlock(&swap_lock);
+	if (ret == 0)
+		try_to_unuse(type, true, pages_to_unuse);
+	return;
+}
+EXPORT_SYMBOL(frontswap_shrink);
+
+/*
+ * Count and return the number of frontswap pages across all
+ * swap devices.  This is exported so that backend drivers can
+ * determine current usage without reading debugfs.
+ */
+unsigned long frontswap_curr_pages(void)
+{
+	unsigned long totalpages = 0;
+
+	spin_lock(&swap_lock);
+	totalpages = __frontswap_curr_pages();
+	spin_unlock(&swap_lock);
+
+	return totalpages;
+}
+EXPORT_SYMBOL(frontswap_curr_pages);
+
+static int __init init_frontswap(void)
+{
+#ifdef CONFIG_DEBUG_FS
+	struct dentry *root = debugfs_create_dir("frontswap", NULL);
+	if (root == NULL)
+		return -ENXIO;
+	debugfs_create_u64("loads", 0444, root, &frontswap_loads);
+	debugfs_create_u64("succ_stores", 0444, root, &frontswap_succ_stores);
+	debugfs_create_u64("failed_stores", 0444, root,
+			   &frontswap_failed_stores);
+	debugfs_create_u64("invalidates", 0444, root, &frontswap_invalidates);
+#endif
+	return 0;
+}
+
+module_init(init_frontswap);