Re: [RFC PATCH v3 3/4] mm/vmscan: avoid pointless large folio splits without swap

[Barry Song]

On Fri, Jul 17, 2026 at 8:25 PM Xueyuan Chen <xueyuan.chen21@xxxxxxxxx> wrote:
>
> From: "Barry Song (Xiaomi)" <baohua@xxxxxxxxxx>
>
> When swap is disabled, exhausted, or unavailable due to memcg swap
> limits, splitting a large anonymous folio cannot make swapout progress.
> The fallback only destroys the large folio and inflates split statistics.
>
> Use -E2BIG from folio_alloc_swap() as the explicit signal that splitting
> the folio might allow swapout of smaller pieces. For other allocation
> failures, keep the existing activation path and avoid the split.
>
> This preserves the split fallback for fragmented or partially available
> swap, while avoiding it when there is no backing space for any part of the
> folio.
>
> Reported-by: Nanzhe Zhao <zhaonanzhe@xxxxxxxxxx>
> Signed-off-by: Barry Song (Xiaomi) <baohua@xxxxxxxxxx>
> ---
>  mm/vmscan.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index bd1b1aa12581..40340a88f78e 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1260,6 +1260,8 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
>                  */
>                 if (folio_test_anon(folio) && folio_test_swapbacked(folio) &&
>                                 !folio_test_swapcache(folio)) {
> +                       int ret;
> +
>                         if (!(sc->gfp_mask & __GFP_IO))
>                                 goto keep_locked;
>                         if (folio_maybe_dma_pinned(folio))
> @@ -1278,10 +1280,11 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
>                                     split_folio_to_list(folio, folio_list))
>                                         goto activate_locked;
>                         }
> -                       if (folio_alloc_swap(folio)) {
> +                       ret = folio_alloc_swap(folio);
> +                       if (ret) {
>                                 int __maybe_unused order = folio_order(folio);
>
> -                               if (!folio_test_large(folio))
> +                               if (!folio_test_large(folio) || ret != -E2BIG)

I'd rather split it into two checks:

if (!folio_test_large(folio))
         goto activate_locked_split;

if (ret != -E2BIG)
         goto activate_locked_split;

This makes it clear that ret != -E2BIG is only relevant for the large
folio case.


>                                         goto activate_locked_split;
>                                 /* Fallback to swap normal pages */
>                                 if (split_folio_to_list(folio, folio_list))
> --
> 2.47.3
>

Best Regards
Barry