Re: [RFC PATCH v2 3/3] mm/vmscan: avoid pointless large folio splits without swap

On 7/9/26 10:51 PM, Xueyuan Chen wrote:
> From: "Barry Song (Xiaomi)" <[email protected]>
> 
> 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 <[email protected]>
> Signed-off-by: Barry Song (Xiaomi) <[email protected]>
> ---
>   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)
>   					goto activate_locked_split;

Like I said in v1 [1], please apply the same change to shmem swap as well.

[1] 
https://lore.kernel.org/all/[email protected]/