mm/shmem, swap: remove redundant error handling for replacing folio

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.

Link: https://lkml.kernel.org/r/20250916160100.31545-10-ryncsn@gmail.com
Signed-off-by: Kairui Song <kasong@tencent.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Suggested-by: Chris Li <chrisl@kernel.org>
Cc: Baoquan He <bhe@redhat.com>
Cc: Barry Song <baohua@kernel.org>
Cc: "Huang, Ying" <ying.huang@linux.alibaba.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: kernel test robot <oliver.sang@intel.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Yosry Ahmed <yosryahmed@google.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/shmem.c b/mm/shmem.c
index 077744a9e9da3814f5f8fe522a86a2ea4cab0d49..dc17717e5631bb64137b05097ff8a3a96bc9ffe9 100644 (file)
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -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) != old);
                xas_next(&xas);
        }
-       if (!error) {
-               mem_cgroup_replace_folio(old, new);
-               shmem_update_stats(new, nr_pages);
-               shmem_update_stats(old, -nr_pages);
-       }
+
+       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;
-       }
+       folio_add_lru(new);
+       *foliop = new;
 
        folio_clear_swapcache(old);
        old->private = NULL;