Re: [PATCH 1/4] mm: swap: move nr_swap_pages counter decrement from folio_alloc_swap() to swap_range_alloc()

[Kairui Song <ryncsn@gmail.com>]

On Thu, May 22, 2025 at 11:32 AM Kemeng Shi <shikemeng@huaweicloud.com> wrote:
>
> When folio_alloc_swap() encounters a failure in either
> mem_cgroup_try_charge_swap() or add_to_swap_cache(), nr_swap_pages counter
> is not decremented for allocated entry. However, the following
> put_swap_folio() will increase nr_swap_pages counter unpairly and lead to
> an imbalance.
>
> Move nr_swap_pages decrement from folio_alloc_swap() to swap_range_alloc()
> to pair the nr_swap_pages counting.
>
> Fixes: 0ff67f990bd45 ("mm, swap: remove swap slot cache")
> Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
> ---
>  mm/swapfile.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 026090bf3efe..75b69213c2e7 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -1115,6 +1115,7 @@ static void swap_range_alloc(struct swap_info_struct *si,
>                 if (vm_swap_full())
>                         schedule_work(&si->reclaim_work);
>         }
> +       atomic_long_sub(nr_entries, &nr_swap_pages);
>  }
>
>  static void swap_range_free(struct swap_info_struct *si, unsigned long offset,
> @@ -1313,7 +1314,6 @@ int folio_alloc_swap(struct folio *folio, gfp_t gfp)
>         if (add_to_swap_cache(folio, entry, gfp | __GFP_NOMEMALLOC, NULL))
>                 goto out_free;
>
> -       atomic_long_sub(size, &nr_swap_pages);
>         return 0;
>
>  out_free:
> --
> 2.30.0

Good catch! Moving the counter update to swap_range_alloc makes the
logic much easier to follow.

Reviewed-by: Kairui Song <kasong@tencent.com>