Re: [PATCH 4/8] mm/swap: Use PG_dropbehind instead of PG_reclaim

[Yosry Ahmed]

On Mon, Jan 13, 2025 at 1:35 AM Kirill A. Shutemov
<kirill.shutemov@xxxxxxxxxxxxxxx> wrote:
>
> The recently introduced PG_dropbehind allows for freeing folios
> immediately after writeback. Unlike PG_reclaim, it does not need vmscan
> to be involved to get the folio freed.
>
> Instead of using folio_set_reclaim(), use folio_set_dropbehind() in
> lru_deactivate_file().
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> ---
>  mm/swap.c | 8 +-------
>  1 file changed, 1 insertion(+), 7 deletions(-)
>
> diff --git a/mm/swap.c b/mm/swap.c
> index fc8281ef4241..4eb33b4804a8 100644
> --- a/mm/swap.c
> +++ b/mm/swap.c
> @@ -562,14 +562,8 @@ static void lru_deactivate_file(struct lruvec *lruvec, struct folio *folio)
>         folio_clear_referenced(folio);
>
>         if (folio_test_writeback(folio) || folio_test_dirty(folio)) {
> -               /*
> -                * Setting the reclaim flag could race with
> -                * folio_end_writeback() and confuse readahead.  But the
> -                * race window is _really_ small and  it's not a critical
> -                * problem.
> -                */
>                 lruvec_add_folio(lruvec, folio);
> -               folio_set_reclaim(folio);
> +               folio_set_dropbehind(folio);
>         } else {
>                 /*
>                  * The folio's writeback ended while it was in the batch.

Now there's a difference in behavior here depending on whether or not
the folio is under writeback (or will be written back soon). If it is,
we set PG_dropbehind to get it freed right after, but if writeback has
already ended we put it on the tail of the LRU to be freed later.

It's a bit counterintuitive to me that folios with pending writeback
get freed faster than folios that completed their writeback already.
Am I missing something?