linux-kernel - Re: [PATCH v3] mm: Fix for negative counter: nr_file

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Date:   Mon, 13 Nov 2023 17:42:37 -0800
From:   Yang Shi <shy828301@...il.com>
To:     Stefan Roesch <shr@...kernel.io>
Cc:     kernel-team@...com, akpm@...ux-foundation.org, hannes@...xchg.org,
        riel@...riel.com, linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        stable@...r.kernel.org, willy@...radead.org
Subject: Re: [PATCH v3] mm: Fix for negative counter: nr_file_hugepages

On Wed, Nov 8, 2023 at 9:15 AM Stefan Roesch <shr@...kernel.io> wrote:
>
> While qualifiying the 6.4 release, the following warning was detected in
> messages:
>
> vmstat_refresh: nr_file_hugepages -15664
>
> The warning is caused by the incorrect updating of the NR_FILE_THPS
> counter in the function split_huge_page_to_list. The if case is checking
> for folio_test_swapbacked, but the else case is missing the check for
> folio_test_pmd_mappable. The other functions that manipulate the counter
> like __filemap_add_folio and filemap_unaccount_folio have the
> corresponding check.
>
> I have a test case, which reproduces the problem. It can be found here:
>   https://github.com/sroeschus/testcase/blob/main/vmstat_refresh/madv.c
>
> The test case reproduces on an XFS filesystem. Running the same test
> case on a BTRFS filesystem does not reproduce the problem.
>
> AFAIK version 6.1 until 6.6 are affected by this problem.
>
> Signed-off-by: Stefan Roesch <shr@...kernel.io>
> Co-debugged-by: Johannes Weiner <hannes@...xchg.org>
> Acked-by: Johannes Weiner <hannes@...xchg.org>
> Cc: stable@...r.kernel.org

Reviewed-by: Yang Shi <shy828301@...il.com>

> ---
>  mm/huge_memory.c | 16 +++++++++-------
>  1 file changed, 9 insertions(+), 7 deletions(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 064fbd90822b4..874000f97bfc1 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2737,13 +2737,15 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
>                         int nr = folio_nr_pages(folio);
>
>                         xas_split(&xas, folio, folio_order(folio));
> -                       if (folio_test_swapbacked(folio)) {
> -                               __lruvec_stat_mod_folio(folio, NR_SHMEM_THPS,
> -                                                       -nr);
> -                       } else {
> -                               __lruvec_stat_mod_folio(folio, NR_FILE_THPS,
> -                                                       -nr);
> -                               filemap_nr_thps_dec(mapping);
> +                       if (folio_test_pmd_mappable(folio)) {
> +                               if (folio_test_swapbacked(folio)) {
> +                                       __lruvec_stat_mod_folio(folio,
> +                                                       NR_SHMEM_THPS, -nr);
> +                               } else {
> +                                       __lruvec_stat_mod_folio(folio,
> +                                                       NR_FILE_THPS, -nr);
> +                                       filemap_nr_thps_dec(mapping);
> +                               }
>                         }
>                 }
>
>
> base-commit: ffc253263a1375a65fa6c9f62a893e9767fbebfa
> --
> 2.39.3
>
>