Re: [PATCH v5 2/3] mm: swap: drop dropbehind swap cache folios on writeback completion

[Barry Song]

On Fri, Sep 11, 2026 at 8:16 PM Alexandre Ghiti <alex@xxxxxxxx> wrote:
>
> A PG_dropbehind folio is dropped from its cache once writeback completes
> rather than left for reclaim to find later; this is implemented for file
> folios in folio_end_dropbehind(). Extend it to swap cache folios.
>
> The drop takes the folio and swap cluster locks and may sleep, so it
> cannot run in interrupt context. Set BIO_COMPLETE_IN_TASK on the write,
> as the file dropbehind paths do, and drop the folio directly from
> folio_end_writeback().
>
> Suggested-by: Yosry Ahmed <yosry@xxxxxxxxxx>
> Suggested-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> Suggested-by: Nhat Pham <nphamcs@xxxxxxxxx>
> Reviewed-by: Nhat Pham <nphamcs@xxxxxxxxx>
> Reviewed-by: Kunwu Chan <kunwu.chan@xxxxxxxxx>
> Signed-off-by: Alexandre Ghiti <alex@xxxxxxxx>
> ---
[...]

> diff --git a/mm/page_io.c b/mm/page_io.c
> index 88962571cb93..52eae99de6e3 100644
> --- a/mm/page_io.c
> +++ b/mm/page_io.c
> @@ -602,6 +602,15 @@ static void swap_bdev_submit_write(struct swap_io_ctx *ctx)
>                 submit_bio_wait(bio);
>                 end_swap_bio_write(bio);
>         } else {
> +               int p;
> +
> +               for (p = 0; p < sio->nr_bvecs; p++) {
> +                       if (folio_test_dropbehind(bvec_folio(&sio->bvecs[p]))) {
> +                               bio_set_flag(bio, BIO_COMPLETE_IN_TASK);

Thanks very much, Alexandre. I find the code is really much,
much better, but I’m still struggling to understand why we need a
sleepable context after Wenjian’s commit:

https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git/commit/?id=848d2ce2fce15fbdc083fbf9691bfa72911033c4

With this question:

https://lore.kernel.org/linux-mm/CAGsJ_4wj6XvPB+QF+_X1BJ_8Y2sZEEFAw6RaJJz-cHaDWLkxaQ@xxxxxxxxxxxxxx/T/#u

In short, it still feels a little strange to me that we need a
sleepable context, especially now that we no longer call
`folio_unmap_invalidate()` for unmapped folios.

Best Regards
Barry