Re: [PATCH hotfix] mm: shmem: fix ShmemHugePages at swapout

[Yosry Ahmed]

On Wed, Dec 4, 2024 at 10:50 PM Hugh Dickins <hughd@xxxxxxxxxx> wrote:
>
> /proc/meminfo ShmemHugePages has been showing overlarge amounts (more
> than Shmem) after swapping out THPs: we forgot to update NR_SHMEM_THPS.
>
> Add shmem_update_stats(), to avoid repetition, and risk of making that
> mistake again: the call from shmem_delete_from_page_cache() is the bugfix;
> the call from shmem_replace_folio() is reassuring, but not really a bugfix
> (replace corrects misplaced swapin readahead, but huge swapin readahead
> would be a mistake).
>
> Fixes: 809bc86517cc ("mm: shmem: support large folio swap out")
> Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>

Reviewed-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx>

> Cc: stable@xxxxxxxxxxxxxxx
> ---
>  mm/shmem.c | 22 ++++++++++++----------
>  1 file changed, 12 insertions(+), 10 deletions(-)
>
> diff --git a/mm/shmem.c b/mm/shmem.c
> index ccb9629a0f70..f6fb053ac50d 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -787,6 +787,14 @@ static bool shmem_huge_global_enabled(struct inode *inode, pgoff_t index,
>  }
>  #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
>
> +static void shmem_update_stats(struct folio *folio, int nr_pages)
> +{
> +       if (folio_test_pmd_mappable(folio))
> +               __lruvec_stat_mod_folio(folio, NR_SHMEM_THPS, nr_pages);
> +       __lruvec_stat_mod_folio(folio, NR_FILE_PAGES, nr_pages);
> +       __lruvec_stat_mod_folio(folio, NR_SHMEM, nr_pages);
> +}
> +
>  /*
>   * Somewhat like filemap_add_folio, but error if expected item has gone.
>   */
> @@ -821,10 +829,7 @@ static int shmem_add_to_page_cache(struct folio *folio,
>                 xas_store(&xas, folio);
>                 if (xas_error(&xas))
>                         goto unlock;
> -               if (folio_test_pmd_mappable(folio))
> -                       __lruvec_stat_mod_folio(folio, NR_SHMEM_THPS, nr);
> -               __lruvec_stat_mod_folio(folio, NR_FILE_PAGES, nr);
> -               __lruvec_stat_mod_folio(folio, NR_SHMEM, nr);
> +               shmem_update_stats(folio, nr);
>                 mapping->nrpages += nr;
>  unlock:
>                 xas_unlock_irq(&xas);
> @@ -852,8 +857,7 @@ static void shmem_delete_from_page_cache(struct folio *folio, void *radswap)
>         error = shmem_replace_entry(mapping, folio->index, folio, radswap);
>         folio->mapping = NULL;
>         mapping->nrpages -= nr;
> -       __lruvec_stat_mod_folio(folio, NR_FILE_PAGES, -nr);
> -       __lruvec_stat_mod_folio(folio, NR_SHMEM, -nr);
> +       shmem_update_stats(folio, -nr);
>         xa_unlock_irq(&mapping->i_pages);
>         folio_put_refs(folio, nr);
>         BUG_ON(error);
> @@ -1969,10 +1973,8 @@ static int shmem_replace_folio(struct folio **foliop, gfp_t gfp,
>         }
>         if (!error) {
>                 mem_cgroup_replace_folio(old, new);
> -               __lruvec_stat_mod_folio(new, NR_FILE_PAGES, nr_pages);
> -               __lruvec_stat_mod_folio(new, NR_SHMEM, nr_pages);
> -               __lruvec_stat_mod_folio(old, NR_FILE_PAGES, -nr_pages);
> -               __lruvec_stat_mod_folio(old, NR_SHMEM, -nr_pages);
> +               shmem_update_stats(new, nr_pages);
> +               shmem_update_stats(old, -nr_pages);
>         }
>         xa_unlock_irq(&swap_mapping->i_pages);
>
> --
> 2.43.0
>