[PATCH v5 07/19] mm/shmem: never bypass the swap cache for SWP_SYNCHRONOUS_IO

Kairui Song
2025-12-19T19:58:08Z

Now the overhead of the swap cache is trivial to none, bypassing the
swap cache is no longer a good optimization.

We have removed the cache bypass swapin for anon memory, now do the same
for shmem. Many helpers and functions can be dropped now.

The performance may slightly drop because of the co-existence and double
update of swap_map and swap table, and this problem will be improved
very soon in later commits by dropping the swap_map update partially:

Swapin of 24 GB file with tmpfs with
transparent_hugepage_tmpfs=within_size and ZRAM, 3 test runs on my
machine:

Before: After this commit: After this series:
5.99s 6.29s 6.08s

And later swap table phases will drop the swap_map completely to avoid
overhead and reduce memory usage.

Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Tested-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Signed-off-by: Kairui Song <kasong@tencent.com>
---
mm/shmem.c | 65 +++++++++++++++++------------------------------------------
mm/swap.h | 4 ----
mm/swapfile.c | 35 +++++++++-----------------------
3 files changed, 27 insertions(+), 77 deletions(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index dd136d40631c..d7eeeaa9580d 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2014,10 +2014,9 @@ static struct folio *shmem_swap_alloc_folio(struct inode *inode,
swp_entry_t entry, int order, gfp_t gfp)
{
struct shmem_inode_info *info = SHMEM_I(inode);
+ struct folio *new, *swapcache;
int nr_pages = 1 << order;
- struct folio *new;
gfp_t alloc_gfp;
- void *shadow;

/*
* We have arrived here because our zones are constrained, so don't
@@ -2057,34 +2056,19 @@ static struct folio *shmem_swap_alloc_folio(struct inode *inode,
goto fallback;
}

- /*
- * Prevent parallel swapin from proceeding with the swap cache flag.
- *
- * Of course there is another possible concurrent scenario as well,
- * that is to say, the swap cache flag of a large folio has already
- * been set by swapcache_prepare(), while another thread may have
- * already split the large swap entry stored in the shmem mapping.
- * In this case, shmem_add_to_page_cache() will help identify the
- * concurrent swapin and return -EEXIST.
- */
- if (swapcache_prepare(entry, nr_pages)) {
+ swapcache = swapin_folio(entry, new);
+ if (swapcache != new) {
folio_put(new);
- new = ERR_PTR(-EEXIST);
- /* Try smaller folio to avoid cache conflict */
- goto fallback;
+ if (!swapcache) {
+ /*
+ * The new folio is charged already, swapin can
+ * only fail due to another raced swapin.
+ */
+ new = ERR_PTR(-EEXIST);
+ goto fallback;
+ }
}
-
- __folio_set_locked(new);
- __folio_set_swapbacked(new);
- new->swap = entry;
-
- memcg1_swapin(entry, nr_pages);
- shadow = swap_cache_get_shadow(entry);
- if (shadow)
- workingset_refault(new, shadow);
- folio_add_lru(new);
- swap_read_folio(new, NULL);
- return new;
+ return swapcache;
fallback:
/* Order 0 swapin failed, nothing to fallback to, abort */
if (!order)
@@ -2174,8 +2158,7 @@ static int shmem_replace_folio(struct folio **foliop, gfp_t gfp,
}

static void shmem_set_folio_swapin_error(struct inode *inode, pgoff_t index,
- struct folio *folio, swp_entry_t swap,
- bool skip_swapcache)
+ struct folio *folio, swp_entry_t swap)
{
struct address_space *mapping = inode->i_mapping;
swp_entry_t swapin_error;
@@ -2191,8 +2174,7 @@ static void shmem_set_folio_swapin_error(struct inode *inode, pgoff_t index,

nr_pages = folio_nr_pages(folio);
folio_wait_writeback(folio);
- if (!skip_swapcache)
- swap_cache_del_folio(folio);
+ swap_cache_del_folio(folio);
/*
* Don't treat swapin error folio as alloced. Otherwise inode->i_blocks
* won't be 0 when inode is released and thus trigger WARN_ON(i_blocks)
@@ -2292,7 +2274,6 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
softleaf_t index_entry;
struct swap_info_struct *si;
struct folio *folio = NULL;
- bool skip_swapcache = false;
int error, nr_pages, order;
pgoff_t offset;

@@ -2335,7 +2316,6 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
folio = NULL;
goto failed;
}
- skip_swapcache = true;
} else {
/* Cached swapin only supports order 0 folio */
folio = shmem_swapin_cluster(swap, gfp, info, index);
@@ -2391,9 +2371,8 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
* and swap cache folios are never partially freed.
*/
folio_lock(folio);
- if ((!skip_swapcache && !folio_test_swapcache(folio)) ||
- shmem_confirm_swap(mapping, index, swap) < 0 ||
- folio->swap.val != swap.val) {
+ if (!folio_matches_swap_entry(folio, swap) ||
+ shmem_confirm_swap(mapping, index, swap) < 0) {
error = -EEXIST;
goto unlock;
}
@@ -2425,12 +2404,7 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
if (sgp == SGP_WRITE)
folio_mark_accessed(folio);

- if (skip_swapcache) {
- folio->swap.val = 0;
- swapcache_clear(si, swap, nr_pages);
- } else {
- swap_cache_del_folio(folio);
- }
+ swap_cache_del_folio(folio);
folio_mark_dirty(folio);
swap_free_nr(swap, nr_pages);
put_swap_device(si);
@@ -2441,14 +2415,11 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
if (shmem_confirm_swap(mapping, index, swap) < 0)
error = -EEXIST;
if (error == -EIO)
- shmem_set_folio_swapin_error(inode, index, folio, swap,
- skip_swapcache);
+ shmem_set_folio_swapin_error(inode, index, folio, swap);
unlock:
if (folio)
folio_unlock(folio);
failed_nolock:
- if (skip_swapcache)
- swapcache_clear(si, folio->swap, folio_nr_pages(folio));
if (folio)
folio_put(folio);
put_swap_device(si);
diff --git a/mm/swap.h b/mm/swap.h
index 214e7d041030..e0f05babe13a 100644
--- a/mm/swap.h
+++ b/mm/swap.h
@@ -403,10 +403,6 @@ static inline int swap_writeout(struct folio *folio,
return 0;
}

-static inline void swapcache_clear(struct swap_info_struct *si, swp_entry_t entry, int nr)
-{
-}
-
static inline struct folio *swap_cache_get_folio(swp_entry_t entry)
{
return NULL;
diff --git a/mm/swapfile.c b/mm/swapfile.c
index e5284067a442..3762b8f3f9e9 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -1614,22 +1614,6 @@ struct swap_info_struct *get_swap_device(swp_entry_t entry)
return NULL;
}

-static void swap_entries_put_cache(struct swap_info_struct *si,
- swp_entry_t entry, int nr)
-{
- unsigned long offset = swp_offset(entry);
- struct swap_cluster_info *ci;
-
- ci = swap_cluster_lock(si, offset);
- if (swap_only_has_cache(si, offset, nr)) {
- swap_entries_free(si, ci, entry, nr);
- } else {
- for (int i = 0; i < nr; i++, entry.val++)
- swap_entry_put_locked(si, ci, entry, SWAP_HAS_CACHE);
- }
- swap_cluster_unlock(ci);
-}
-
static bool swap_entries_put_map(struct swap_info_struct *si,
swp_entry_t entry, int nr)
{
@@ -1765,13 +1749,21 @@ void swap_free_nr(swp_entry_t entry, int nr_pages)
void put_swap_folio(struct folio *folio, swp_entry_t entry)
{
struct swap_info_struct *si;
+ struct swap_cluster_info *ci;
+ unsigned long offset = swp_offset(entry);
int size = 1 << swap_entry_order(folio_order(folio));

si = _swap_info_get(entry);
if (!si)
return;

- swap_entries_put_cache(si, entry, size);
+ ci = swap_cluster_lock(si, offset);
+ if (swap_only_has_cache(si, offset, size))
+ swap_entries_free(si, ci, entry, size);
+ else
+ for (int i = 0; i < size; i++, entry.val++)
+ swap_entry_put_locked(si, ci, entry, SWAP_HAS_CACHE);
+ swap_cluster_unlock(ci);
}

int __swap_count(swp_entry_t entry)
@@ -3784,15 +3776,6 @@ int swapcache_prepare(swp_entry_t entry, int nr)
return __swap_duplicate(entry, SWAP_HAS_CACHE, nr);
}

-/*
- * Caller should ensure entries belong to the same folio so
- * the entries won't span cross cluster boundary.
- */
-void swapcache_clear(struct swap_info_struct *si, swp_entry_t entry, int nr)
-{
- swap_entries_put_cache(si, entry, nr);
-}
-
/*
* add_swap_count_continuation - called when a swap count is duplicated
* beyond SWAP_MAP_MAX, it allocates a new page and links that to the entry's

--
2.52.0