/*
* 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
*
* 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>
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) {
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.
*
* 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
* 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 necessory and feasible to unuse pages.
* Return 1 when nothing to do, 0 when need to shink 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 uninitialized_var(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);
