[PATCH v3 09/15] mm/shmem, swap: remove redundant error handling for replacing folio

Kairui Song
2025-09-10T16:09:35Z

From: Kairui Song <kasong@tencent.com>

Shmem may replace a folio in the swap cache if the cached one doesn't
fit the swapin's GFP zone. When doing so, shmem has already double
checked that the swap cache folio is locked, still has the swap cache
flag set, and contains the wanted swap entry. So it is impossible to
fail due to an XArray mismatch. There is even a comment for that.

Delete the defensive error handling path, and add a WARN_ON instead:
if that happened, something has broken the basic principle of how the
swap cache works, we should catch and fix that.

Signed-off-by: Kairui Song <kasong@tencent.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
---
mm/shmem.c | 42 ++++++++++++------------------------------
1 file changed, 12 insertions(+), 30 deletions(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index 410f27bc4752..5f395fab489c 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1661,13 +1661,13 @@ int shmem_writeout(struct folio *folio, struct swap_iocb **plug,
}

/*
- * The delete_from_swap_cache() below could be left for
+ * The swap_cache_del_folio() below could be left for
* shrink_folio_list()'s folio_free_swap() to dispose of;
* but I'm a little nervous about letting this folio out of
* shmem_writeout() in a hybrid half-tmpfs-half-swap state
* e.g. folio_mapping(folio) might give an unexpected answer.
*/
- delete_from_swap_cache(folio);
+ swap_cache_del_folio(folio);
goto redirty;
}
if (nr_pages > 1)
@@ -2045,7 +2045,7 @@ static struct folio *shmem_swap_alloc_folio(struct inode *inode,
new->swap = entry;

memcg1_swapin(entry, nr_pages);
- shadow = get_shadow_from_swap_cache(entry);
+ shadow = swap_cache_get_shadow(entry);
if (shadow)
workingset_refault(new, shadow);
folio_add_lru(new);
@@ -2121,35 +2121,17 @@ static int shmem_replace_folio(struct folio **foliop, gfp_t gfp,
/* Swap cache still stores N entries instead of a high-order entry */
xa_lock_irq(&swap_mapping->i_pages);
for (i = 0; i < nr_pages; i++) {
- void *item = xas_load(&xas);
-
- if (item != old) {
- error = -ENOENT;
- break;
- }
-
- xas_store(&xas, new);
+ WARN_ON_ONCE(xas_store(&xas, new));
xas_next(&xas);
}
- if (!error) {
- mem_cgroup_replace_folio(old, new);
- shmem_update_stats(new, nr_pages);
- shmem_update_stats(old, -nr_pages);
- }
xa_unlock_irq(&swap_mapping->i_pages);

- if (unlikely(error)) {
- /*
- * Is this possible? I think not, now that our callers
- * check both the swapcache flag and folio->private
- * after getting the folio lock; but be defensive.
- * Reverse old to newpage for clear and free.
- */
- old = new;
- } else {
- folio_add_lru(new);
- *foliop = new;
- }
+ mem_cgroup_replace_folio(old, new);
+ shmem_update_stats(new, nr_pages);
+ shmem_update_stats(old, -nr_pages);
+
+ folio_add_lru(new);
+ *foliop = new;

folio_clear_swapcache(old);
old->private = NULL;
@@ -2183,7 +2165,7 @@ 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)
- delete_from_swap_cache(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)
@@ -2422,7 +2404,7 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
folio->swap.val = 0;
swapcache_clear(si, swap, nr_pages);
} else {
- delete_from_swap_cache(folio);
+ swap_cache_del_folio(folio);
}
folio_mark_dirty(folio);
swap_free_nr(swap, nr_pages);
--
2.51.0