Re: [PATCH] mm/huge_memory: update file PMD counter before folio_put()

[Lorenzo Stoakes]

On Tue, May 26, 2026 at 06:13:37PM +0800, Yin Tirui wrote:
> __split_huge_pmd_locked() updates the file/shmem RSS counter after
> dropping the PMD mapping's folio reference. If folio_put() drops the
> last reference, mm_counter_file() can later read freed folio state via
> folio_test_swapbacked().
>
> Move the counter update before folio_put().
>
> Fixes: fadae2953072 ("thp: use mm_file_counter to determine update which rss counter")

That's an old commit :) I mean I suspect we're probably not actually ever
dropping the folio ref to 0 here since we never had a report since ~2018.

The page cache keeping a reference I guess?

But doesn't mean we shouldn't fix this on principal/there being some way
this could happen.

> Cc: <stable@xxxxxxxxxxxxxxx>
> Signed-off-by: Yin Tirui <yintirui@xxxxxxxxxx>

LGTM, so:

Reviewed-by: Lorenzo Stoakes <ljs@xxxxxxxxxx>

> ---
>  mm/huge_memory.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 0135c29a4372..a5f4a48b7b77 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -3145,7 +3145,9 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
>  			if (!folio_test_referenced(folio) && pmd_young(old_pmd))
>  				folio_set_referenced(folio);
>  			folio_remove_rmap_pmd(folio, page, vma);
> +			add_mm_counter(mm, mm_counter_file(folio), -HPAGE_PMD_NR);
>  			folio_put(folio);
> +			return;

Hmm, sucks to duplicate like this, but for purposes of backport and getting
this resolved fine, we can clean it up later.

>  		}
>  		add_mm_counter(mm, mm_counter_file(folio), -HPAGE_PMD_NR);
>  		return;
> --
> 2.43.0
>

Cheers, Lorenzo