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/*
* 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;

/*
* This global enablement flag reduces overhead on systems where frontswap_ops
* has not been registered, so is preferred to the slower alternative: a
* function call that checks a non-global.
*/
bool frontswap_enabled __read_mostly;
EXPORT_SYMBOL(frontswap_enabled);

/*
* 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;

#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
/*
* 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;

frontswap_ops = *ops;
frontswap_enabled = true;
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);

/*
* Called when a swap device is swapon'd.
*/
void __frontswap_init(unsigned type)
{
struct swap_info_struct *sis = swap_info[type];

BUG_ON(sis == NULL);
if (sis->frontswap_map == NULL)
return;
frontswap_ops.init(type);
}
EXPORT_SYMBOL(__frontswap_init);

static inline void __frontswap_clear(struct swap_info_struct *sis, pgoff_t offset)
{
frontswap_clear(sis, offset);
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);

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) {
frontswap_set(sis, offset);
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);
}
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);
if (frontswap_test(sis, offset))
ret = frontswap_ops.load(type, offset, page);
if (ret == 0)
inc_frontswap_loads();
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);
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];

BUG_ON(sis == NULL);
if (sis->frontswap_map == NULL)
return;
frontswap_ops.invalidate_area(type);
atomic_set(&sis->frontswap_pages, 0);
memset(sis->frontswap_map, 0, sis->max / sizeof(long));
}
EXPORT_SYMBOL(__frontswap_invalidate_area);

static unsigned long __frontswap_curr_pages(void)
{
int type;
unsigned long totalpages = 0;
struct swap_info_struct *si = NULL;

assert_spin_locked(&swap_lock);
for (type = swap_list.head; type >= 0; type = si->next) {
si = swap_info[type];
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;
int type;

assert_spin_locked(&swap_lock);
for (type = swap_list.head; type >= 0; type = si->next) {
si = swap_info[type];
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 = type;
ret = 0;
break;
}

return ret;
}

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 0;
}
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_list.head each time
*/
spin_lock(&swap_lock);
ret = __frontswap_shrink(target_pages, &pages_to_unuse, &type);
spin_unlock(&swap_lock);
if (ret == 0 && pages_to_unuse)
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", S_IRUGO, root, &frontswap_loads);
debugfs_create_u64("succ_stores", S_IRUGO, root, &frontswap_succ_stores);
debugfs_create_u64("failed_stores", S_IRUGO, root,
&frontswap_failed_stores);
debugfs_create_u64("invalidates", S_IRUGO,
root, &frontswap_invalidates);
#endif
return 0;
}

module_init(init_frontswap);
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