linux-kernel - Re: [PATCH v1] mm/rmap: cleanup partially-mapped handling in __folio_remove

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Message-ID: <CAJD7tkax7gJdLAMUZ=x7bmDievcLv-w_ridqpTZxC4R-tsAoiw@mail.gmail.com>
Date: Mon, 24 Jun 2024 04:48:17 -0700
From: Yosry Ahmed <yosryahmed@...gle.com>
To: David Hildenbrand <david@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v1] mm/rmap: cleanup partially-mapped handling in __folio_remove_rmap()

On Tue, Jun 18, 2024 at 8:10 AM David Hildenbrand <david@...hat.com> wrote:
>
> Let's simplify and reduce code indentation. In the RMAP_LEVEL_PTE case, we
> already check for nr when computing partially_mapped.
>
> For RMAP_LEVEL_PMD, it's a bit more confusing. Likely, we don't need the
> "nr" check, but we could have "nr < nr_pmdmapped" also if we stumbled
> into the "/* Raced ahead of another remove and an add? */" case. So
> let's simply move the nr check in there.
>
> Note that partially_mapped is always false for small folios.
>
> No functional change intended.
>
> Cc: Yosry Ahmed <yosryahmed@...gle.com>
> Signed-off-by: David Hildenbrand <david@...hat.com>

Makes sense to me. FWIW:

Reviewed-by: Yosry Ahmed <yosryahmed@...gle.com>

> ---
>  mm/rmap.c | 10 ++++------
>  1 file changed, 4 insertions(+), 6 deletions(-)
>
> diff --git a/mm/rmap.c b/mm/rmap.c
> index afec4b6800caf..aa900e46cdf82 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1551,11 +1551,12 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
>                         }
>                 }
>
> -               partially_mapped = nr < nr_pmdmapped;
> +               partially_mapped = nr && nr < nr_pmdmapped;
>                 break;
>         }
>
> -       if (nr) {
> +       if (partially_mapped && folio_test_anon(folio) &&
> +           list_empty(&folio->_deferred_list))
>                 /*
>                  * Queue anon large folio for deferred split if at least one
>                  * page of the folio is unmapped and at least one page
> @@ -1563,10 +1564,7 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
>                  *
>                  * Check partially_mapped first to ensure it is a large folio.
>                  */
> -               if (folio_test_anon(folio) && partially_mapped &&
> -                   list_empty(&folio->_deferred_list))
> -                       deferred_split_folio(folio);
> -       }
> +               deferred_split_folio(folio);

FWIW, I prefer moving the comment out of the one-line if block as you
originally suggested in [1].

[1]https://lore.kernel.org/lkml/1a408ed1-7e81-457e-a205-db274b4d6b78@redhat.com/

>         __folio_mod_stat(folio, -nr, -nr_pmdmapped);
>
>         /*
> --
> 2.45.2
>