Re: [PATCH v4 09/15] mm/shmem, swap: remove redundant error handling for replacing folio

[Chris Li <chrisl@kernel.org>]

On Tue, Sep 16, 2025 at 9:02 AM Kairui Song <ryncsn@gmail.com> wrote:
>
> 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>
> Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>

Acked-by: Chris Li <chrisl@kernel.org>

Chris

> ---
>  mm/shmem.c | 32 +++++++-------------------------
>  1 file changed, 7 insertions(+), 25 deletions(-)
>
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 077744a9e9da..dc17717e5631 100644
> --- 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;
> --
> 2.51.0
>
>