// 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 const struct frontswap_ops *frontswap_ops __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
*/
int frontswap_register_ops(const struct frontswap_ops *ops)
{
if (frontswap_ops)
return -EINVAL;
frontswap_ops = ops;
static_branch_inc(&frontswap_enabled_key);
return 0;
}
/*
* Called when a swap device is swapon'd.
*/
void frontswap_init(unsigned type, unsigned long *map)
{
struct swap_info_struct *sis = swap_info[type];
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);
frontswap_ops->init(type);
}
static bool __frontswap_test(struct swap_info_struct *sis,
pgoff_t offset)
{
if (sis->frontswap_map)
return test_bit(offset, sis->frontswap_map);
return false;
}
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);
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);
frontswap_ops->invalidate_page(type, offset);
}
ret = frontswap_ops->store(type, offset, page);
if (ret == 0) {
__frontswap_set(sis, offset);
inc_frontswap_succ_stores();
} else {
inc_frontswap_failed_stores();
}
return ret;
}
/*
* "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);
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. */
ret = frontswap_ops->load(type, offset, page);
if (ret == 0)
inc_frontswap_loads();
return ret;
}
/*
* 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];
VM_BUG_ON(!frontswap_ops);
VM_BUG_ON(sis == NULL);
if (!__frontswap_test(sis, offset))
return;
frontswap_ops->invalidate_page(type, offset);
__frontswap_clear(sis, offset);
inc_frontswap_invalidates();
}
/*
* 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];
VM_BUG_ON(!frontswap_ops);
VM_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);
}
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);
