Re: [PATCH v4 2/3] mm: swap: drop dropbehind swap cache folios on writeback completion

[Nhat Pham]

On Tue, Aug 25, 2026 at 6:54 AM 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().

This BIO_COMPLETE_IN_TASK thing is really nice! I'm glad we have a
proper infra now and no longer have to roll our own solutions :)

>
> Suggested-by: Yosry Ahmed <yosry@xxxxxxxxxx>
> Suggested-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> Suggested-by: Nhat Pham <nphamcs@xxxxxxxxx>
> Signed-off-by: Alexandre Ghiti <alex@xxxxxxxx>

Mostly LGTM FWIW. Just one query below:

[...]

> +        * Gate remove_mapping_reclaim() on folio_test_swapcache(): a racing
> +        * swapin may have freed the swap slot (folio_free_swap()) and dropped the
> +        * folio from the cache, and it must not run on a non-swapcache folio (it
> +        * would trip __remove_mapping()'s mapping == folio_mapping() check).
> +        */
> +       if (!folio_test_swapcache(folio) || folio_test_writeback(folio) ||
> +           !remove_mapping_reclaim(swap_address_space(folio->swap), folio, memcg)) {

remove_mapping_reclaim() and the likes seem a bit confusing to me, but
I don't have a good suggestions for the naming...

(also I'm hoping we're fixing the weird shadows behavior of zswap
writeback soon :))

Anyway:

Reviewed-by: Nhat Pham <nphamcs@xxxxxxxxx>