Re: [PATCH 1/3] mm/swap: revert to single-folio writes for synchronous swap devices

On Wed, Jul 22, 2026 at 9:06 PM Christoph Hellwig <[email protected]> wrote:
>
> Kairui Song reported that zram benefits from submitting each folio
> directly instead of batching up I/O because the classic LRU scanning
> benefits from clearing the folio writeback bit in the scan loop.
>
> Accommodate that by kicking off reads for synchronous devices for
> each iteration.
>
> Signed-off-by: Christoph Hellwig <[email protected]>
> ---
>  mm/page_io.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
>
> diff --git a/mm/page_io.c b/mm/page_io.c
> index e4fa7ffffe8b..c984a4023a65 100644
> --- a/mm/page_io.c
> +++ b/mm/page_io.c
> @@ -358,7 +358,16 @@ static void swap_add_folio(struct swap_io_ctx *ctx, struct folio *folio, int rw)
>         }
>         bvec_set_folio(&sio->bvecs[sio->nr_bvecs], folio, folio_size(folio), 0);
>         sio->len += folio_size(folio);
> -       if (++sio->nr_bvecs == ARRAY_SIZE(sio->bvecs)) {
> +
> +       /*
> +        * Write out the iocb if we filled it, or if the device is synchronous.
> +        *
> +        * The latter is to work around expectations in the classic LRU code
> +        * which make synchronous clearing of the folio writeback flag in the
> +        * reclaim path beneficial.
> +        */
> +       if (++sio->nr_bvecs == ARRAY_SIZE(sio->bvecs) ||
> +           (rw == WRITE && (sis->flags & SWP_SYNCHRONOUS_IO))) {
>                 if (rw == WRITE)
>                         swap_write_submit(ctx);
>                 else
> --
> 2.53.0
>

Looks good, thanks!

Acked-by: Kairui Song <[email protected]>