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

[Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>]

On 2024/12/5 14:50, Hugh Dickins 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>
> Cc: stable@xxxxxxxxxxxxxxx

Indeed. Thanks for fixing.
Reviewed-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
Tested-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>

> ---
>   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);
>