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mm/shmem, swap: remove SWAP_MAP_SHMEM

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The SWAP_MAP_SHMEM state was introduced in the commit aaa468653b
("swap_info: note SWAP_MAP_SHMEM"), to quickly determine if a swap entry
belongs to shmem during swapoff.

However, swapoff has since been rewritten in the commit b56a2d8af9 ("mm:
rid swapoff of quadratic complexity").  Now having swap count ==
SWAP_MAP_SHMEM value is basically the same as having swap count == 1, and
swap_shmem_alloc() behaves analogously to swap_duplicate().  The only
difference of note is that swap_shmem_alloc() does not check for -ENOMEM
returned from __swap_duplicate(), but it is OK because shmem never
re-duplicates any swap entry it owns.  This will stil be safe if we use
(batched) swap_duplicate() instead.

This commit adds swap_duplicate_nr(), the batched variant of
swap_duplicate(), and removes the SWAP_MAP_SHMEM state and the associated
swap_shmem_alloc() helper to simplify the state machine (both mentally and
in terms of actual code).  We will also have an extra state/special value
that can be repurposed (for swap entries that never gets re-duplicated).

Link: https://lkml.kernel.org/r/20251220-swap-table-p2-v5-8-8862a265a033@tencent.com
Signed-off-by: Kairui Song <kasong@tencent.com>
Signed-off-by: Nhat Pham <nphamcs@gmail.com>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Tested-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Baoquan He <bhe@redhat.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Chris Li <chrisl@kernel.org>
Cc: Rafael J. Wysocki (Intel) <rafael@kernel.org>
Cc: Yosry Ahmed <yosry.ahmed@linux.dev>
Cc: Deepanshu Kartikey <kartikey406@gmail.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kairui Song <ryncsn@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Nhat Pham 2025-12-20 03:43:37 +08:00 committed by Andrew Morton
parent c246d236b1
commit bc617c990e
3 changed files with 25 additions and 34 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

15
include/linux/swap.h
View File

@ -230,7 +230,6 @@ enum {
/* Special value in first swap_map */
#define SWAP_MAP_MAX 0x3e /* Max count */
#define SWAP_MAP_BAD 0x3f /* Note page is bad */
#define SWAP_MAP_SHMEM 0xbf /* Owned by shmem/tmpfs */
/* Special value in each swap_map continuation */
#define SWAP_CONT_MAX 0x7f /* Max count */
@ -458,8 +457,7 @@ bool folio_free_swap(struct folio *folio);
void put_swap_folio(struct folio *folio, swp_entry_t entry);
extern swp_entry_t get_swap_page_of_type(int);
extern int add_swap_count_continuation(swp_entry_t, gfp_t);
extern void swap_shmem_alloc(swp_entry_t, int);
extern int swap_duplicate(swp_entry_t);
extern int swap_duplicate_nr(swp_entry_t entry, int nr);
extern int swapcache_prepare(swp_entry_t entry, int nr);
extern void swap_free_nr(swp_entry_t entry, int nr_pages);
extern void free_swap_and_cache_nr(swp_entry_t entry, int nr);
@ -514,11 +512,7 @@ static inline int add_swap_count_continuation(swp_entry_t swp, gfp_t gfp_mask)
return 0;
}
static inline void swap_shmem_alloc(swp_entry_t swp, int nr)
{
}
static inline int swap_duplicate(swp_entry_t swp)
static inline int swap_duplicate_nr(swp_entry_t swp, int nr_pages)
{
return 0;
}
@ -569,6 +563,11 @@ static inline int add_swap_extent(struct swap_info_struct *sis,
}
#endif /* CONFIG_SWAP */
static inline int swap_duplicate(swp_entry_t entry)
{
return swap_duplicate_nr(entry, 1);
}
static inline void free_swap_and_cache(swp_entry_t entry)
{
free_swap_and_cache_nr(entry, 1);

2
mm/shmem.c
View File

@ -1690,7 +1690,7 @@ int shmem_writeout(struct folio *folio, struct swap_iocb **plug,
spin_unlock(&shmem_swaplist_lock);
}
swap_shmem_alloc(folio->swap, nr_pages);
swap_duplicate_nr(folio->swap, nr_pages);
shmem_delete_from_page_cache(folio, swp_to_radix_entry(folio->swap));
BUG_ON(folio_mapped(folio));

42
mm/swapfile.c
View File

@ -201,7 +201,7 @@ static bool swap_is_last_map(struct swap_info_struct *si,
unsigned char *map_end = map + nr_pages;
unsigned char count = *map;
if (swap_count(count) != 1 && swap_count(count) != SWAP_MAP_SHMEM)
if (swap_count(count) != 1)
return false;
while (++map < map_end) {
@ -1523,12 +1523,6 @@ static unsigned char swap_entry_put_locked(struct swap_info_struct *si,
if (usage == SWAP_HAS_CACHE) {
VM_BUG_ON(!has_cache);
has_cache = 0;
} else if (count == SWAP_MAP_SHMEM) {
/*
* Or we could insist on shmem.c using a special
* swap_shmem_free() and free_shmem_swap_and_cache()...
*/
count = 0;
} else if ((count & ~COUNT_CONTINUED) <= SWAP_MAP_MAX) {
if (count == COUNT_CONTINUED) {
if (swap_count_continued(si, offset, count))
@ -1626,7 +1620,7 @@ static bool swap_entries_put_map(struct swap_info_struct *si,
if (nr <= 1)
goto fallback;
count = swap_count(data_race(si->swap_map[offset]));
if (count != 1 && count != SWAP_MAP_SHMEM)
if (count != 1)
goto fallback;
ci = swap_cluster_lock(si, offset);
@ -1680,12 +1674,10 @@ static bool swap_entries_put_map_nr(struct swap_info_struct *si,
/*
* Check if it's the last ref of swap entry in the freeing path.
* Qualified value includes 1, SWAP_HAS_CACHE or SWAP_MAP_SHMEM.
*/
static inline bool __maybe_unused swap_is_last_ref(unsigned char count)
{
return (count == SWAP_HAS_CACHE) || (count == 1) ||
(count == SWAP_MAP_SHMEM);
return (count == SWAP_HAS_CACHE) || (count == 1);
}
/*
@ -3678,7 +3670,6 @@ static int __swap_duplicate(swp_entry_t entry, unsigned char usage, int nr)
offset = swp_offset(entry);
VM_WARN_ON(nr > SWAPFILE_CLUSTER - offset % SWAPFILE_CLUSTER);
VM_WARN_ON(usage == 1 && nr > 1);
ci = swap_cluster_lock(si, offset);
err = 0;
@ -3738,27 +3729,28 @@ static int __swap_duplicate(swp_entry_t entry, unsigned char usage, int nr)
return err;
}
/*
* Help swapoff by noting that swap entry belongs to shmem/tmpfs
* (in which case its reference count is never incremented).
*/
void swap_shmem_alloc(swp_entry_t entry, int nr)
{
__swap_duplicate(entry, SWAP_MAP_SHMEM, nr);
}
/*
* Increase reference count of swap entry by 1.
/**
* swap_duplicate_nr() - Increase reference count of nr contiguous swap entries
* by 1.
*
* @entry: first swap entry from which we want to increase the refcount.
* @nr: Number of entries in range.
*
* Returns 0 for success, or -ENOMEM if a swap_count_continuation is required
* but could not be atomically allocated. Returns 0, just as if it succeeded,
* if __swap_duplicate() fails for another reason (-EINVAL or -ENOENT), which
* might occur if a page table entry has got corrupted.
*
* Note that we are currently not handling the case where nr > 1 and we need to
* add swap count continuation. This is OK, because no such user exists - shmem
* is the only user that can pass nr > 1, and it never re-duplicates any swap
* entry it owns.
*/
int swap_duplicate(swp_entry_t entry)
int swap_duplicate_nr(swp_entry_t entry, int nr)
{
int err = 0;
while (!err && __swap_duplicate(entry, 1, 1) == -ENOMEM)
while (!err && __swap_duplicate(entry, 1, nr) == -ENOMEM)
err = add_swap_count_continuation(entry, GFP_ATOMIC);
return err;
}