Re: [RFC PATCH v3 4/4] mm/shmem: split large folios only on -E2BIG

On Fri, Jul 17, 2026 at 8:26 PM Xueyuan Chen <[email protected]> wrote:
>
> shmem_writeout() currently splits a large folio on every
> folio_alloc_swap() failure. With the refined return-value contract, only
> -E2BIG indicates that splitting might allow smaller folios to be swapped
> out.
>
> Enter the split fallback only for -E2BIG. For -ENOSPC and -ENOMEM,
> redirty and reactivate the folio as before.
>
> Suggested-by: Baolin Wang <[email protected]>
> Signed-off-by: Xueyuan Chen <[email protected]>
> ---
>  mm/shmem.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 3b5dc21b323c..d76812dd3cef 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -1599,6 +1599,7 @@ int shmem_writeout(struct folio *folio, struct swap_iocb **plug,
>         struct shmem_sb_info *sbinfo = SHMEM_SB(inode->i_sb);
>         pgoff_t index;
>         int nr_pages;
> +       int ret;
>         bool split = false;
>
>         if ((info->flags & SHMEM_F_LOCKED) || sbinfo->noswap)
> @@ -1679,7 +1680,8 @@ int shmem_writeout(struct folio *folio, struct swap_iocb **plug,
>                 folio_mark_uptodate(folio);
>         }
>
> -       if (!folio_alloc_swap(folio)) {
> +       ret = folio_alloc_swap(folio);
> +       if (!ret) {
>                 bool first_swapped = shmem_recalc_inode(inode, 0, nr_pages);
>                 int error;
>
> @@ -1732,7 +1734,7 @@ int shmem_writeout(struct folio *folio, struct swap_iocb **plug,
>                 swap_cache_del_folio(folio);
>                 goto redirty;
>         }
> -       if (nr_pages > 1)
> +       if (ret == -E2BIG)

I'd rather keep if (nr_pages > 1 && ret == -E2BIG), as it makes the large
folio case more explicit and easier to understand.

>                 goto try_split;
>  redirty:
>         folio_mark_dirty(folio);


Best Regards
Barry