src/testdir/README.txt

/*
 * Frontswap frontend
 *
 * This code provides the generic "frontend" layer to call a matching
 * "backend" driver implementation of frontswap.  See
 * Documentation/vm/frontswap.txt for more information.
 *
 * Copyright (C) 2009-2012 Oracle Corp.  All rights reserved.
 * Author: Dan Magenheimer
 *
 * This work is licensed under the terms of the GNU GPL, version 2.
 */

#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>

/*
 * frontswap_ops is set by frontswap_register_ops to contain the pointers
 * to the frontswap "backend" implementation functions.
 */
static struct frontswap_ops *frontswap_ops __read_mostly;

/*
 * 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) {
	frontswap_loads++;
}
static inline void inc_frontswap_succ_stores(void) {
	frontswap_succ_stores++;
}
static inline void inc_frontswap_failed_stores(void) {
	frontswap_failed_stores++;
}
static inline void inc_frontswap_invalidates(void) {
	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.
 *
 * Specifically when no backend is registered (nobody called
 * frontswap_register_ops) all calls to frontswap_init (which is done via
 * swapon -> enable_swap_info -> frontswap_init) are registered and remembered
 * (via the setting of need_init bitmap) but fail to create tmem_pools. When a
 * backend registers with frontswap at some later point the previous
 * calls to frontswap_init are executed (by iterating over the need_init
 * bitmap) to create tmem_pools and set the respective poolids. All of that is
 * guarded by us using atomic bit operations on the 'need_init' bitmap.
 *
 * 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).
 * Fortunatly for us, the swapon_mutex has been taked 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
 * ignorning or failing the requests - at which point frontswap_ops
 * would have to be made in some fashion atomic.
 */
static DECLARE_BITMAP(need_init, MAX_SWAPFILES);

/*
 * Register operations for frontswap, returning previous thus allowing
 * detection of multiple backends and possible nesting.
 */
struct frontswap_ops *frontswap_register_ops(struct frontswap_ops *ops)
{
	struct frontswap_ops *old = frontswap_ops;
	int i;

	for (i = 0; i < MAX_SWAPFILES; i++) {
		if (test_and_clear_bit(i, need_init)) {
			struct swap_info_struct *sis = swap_info[i];
			/* __frontswap_init _should_ have set it! */
			if (!sis->frontswap_map)
				return ERR_PTR(-EINVAL);
			ops->init(i);
		}
	}
	/*
	 * We MUST have frontswap_ops set _after_ the frontswap_init's
	 * have been called. Otherwise __frontswap_store might fail. Hence
	 * the barrier to make sure compiler does not re-order us.
	 */
	barrier();
	frontswap_ops = ops;
	return old;
}
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];

	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);
	if (frontswap_ops)
		frontswap_ops->init(type);
	else {
		BUG_ON(type >= MAX_SWAPFILES);
		set_bit(type, need_init);
	}
}
EXPORT_SYMBOL(__frontswap_init);

bool __frontswap_test(struct swap_info_struct *sis,
				pgoff_t offset)
{
	bool ret = false;

	if (frontswap_ops && sis->frontswap_map)
		ret = test_bit(offset, sis->frontswap_map);
	return ret;
}
EXPORT_SYMBOL(__frontswap_test);

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, dup = 0;
	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);

	/*
	 * Return if no backend registed.
	 * Don't need to inc frontswap_failed_stores here.
	 */
	if (!frontswap_ops)
		return ret;

	BUG_ON(!PageLocked(page));
	BUG_ON(sis == NULL);
	if (__frontswap_test(sis, offset))
		dup = 1;
	ret = frontswap_ops->store(type, offset, page);
	if (ret == 0) {
		set_bit(offset, sis->frontswap_map);
		inc_frontswap_succ_stores();
		if (!dup)
			atomic_inc(&sis->frontswap_pages);
	} else {
		/*
		  failed dup always results in automatic invalidate of
		  the (older) page from frontswap
		 */
		inc_frontswap_failed_stores();
		if (dup) {
			__frontswap_clear(sis, offset);
			frontswap_ops->invalidate_page(type, offset);
		}
	}
	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);

	BUG_ON(!PageLocked(page));
	BUG_ON(sis == NULL);
	/*
	 * __frontswap_test() will check whether there is backend registered
	 */
	if (__frontswap_test(sis, offset))
		ret = frontswap_ops->load(type, offset, page);
	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];

	BUG_ON(sis == NULL);
	/*
	 * __frontswap_test() will check whether there is backend registered
	 */
	if (__frontswap_test(sis, offset)) {
		frontswap_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];

	if (frontswap_ops) {
		BUG_ON(sis == NULL);
		if (sis->frontswap_map == NULL)
			return;
		frontswap_ops->invalidate_area(type);
		atomic_set(&sis->frontswap_pages, 0);
		bitmap_zero(sis->frontswap_map, sis->max);
	}
	clear_bit(type, need_init);
}
EXPORT_SYMBOL(__frontswap_invalidate_area);

static unsigned long __frontswap_curr_pages(void)
{
	unsigned long totalpages =
Age	Commit message (Expand)	Author
2022-09-09	patch 9.0.0421: MS-Windows makefiles are inconsistently namedv9.0.0421	K.Takata
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2022-01-29	Update runtime files	Bram Moolenaar
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2016-09-03	patch 7.4.2321v7.4.2321	Bram Moolenaar
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2014-08-29	updated for version 7.4.420v7.4.420	Bram Moolenaar