Re: [PATCH v2] mm/page_io: fix PSWPIN undercount for large folios in sio_read_complete()

[Barry Song <21cnbao@gmail.com>]

On Mon, Mar 30, 2026 at 3:12 PM David Carlier <devnexen@gmail.com> wrote:
>
> sio_read_complete() uses sio->pages to account global PSWPIN vm events,
> but sio->pages tracks the number of bvec entries (folios), not base
> pages. For large folios this undercounts compared to the per-memcg path
> which correctly uses folio_nr_pages(), and compared to the bdev read
> paths which also use folio_nr_pages().
>
> Use sio->len >> PAGE_SHIFT instead, which gives the correct base page
> count since sio->len is accumulated via folio_size(folio).
>
> Fixes: a1a0dfd56f97 ("mm: handle THP in swap_*page_fs()")
> Signed-off-by: David Carlier <devnexen@gmail.com>

The patch seems theoretically correct, but I’m wondering
where we can swap in mTHP for filesystem-based swap？

In both do_swap_page() and shmem_swapin_folio(), we check
data_race(si->flags & SWP_SYNCHRONOUS_IO) before allocating
large folios. Am I missing something?

> ---
>  mm/page_io.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/page_io.c b/mm/page_io.c
> index 63b262f4c5a9..1389cd57ca88 100644
> --- a/mm/page_io.c
> +++ b/mm/page_io.c
> @@ -497,7 +497,7 @@ static void sio_read_complete(struct kiocb *iocb, long ret)
>                         folio_mark_uptodate(folio);
>                         folio_unlock(folio);
>                 }
> -               count_vm_events(PSWPIN, sio->pages);
> +               count_vm_events(PSWPIN, sio->len >> PAGE_SHIFT);
>         } else {
>                 for (p = 0; p < sio->pages; p++) {
>                         struct folio *folio = page_folio(sio->bvec[p].bv_page);
> --
> 2.53.0
>

Thanks
Barry