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

David Hildenbrand (Arm)
2026-03-30T06:38:00Z

Hi,

please don't send patches a reply to other patches.

On 3/24/26 00:13, David Carlier 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: 6a8c068774ad ("mm/page_io: use a folio in sio_read_complete()")

Is that commit really to blame? I don't see how that one would cause it.
It would rather be the code that actually made more than one page be
represented in a single entry.

Is maybe a1a0dfd56f97 ("mm: handle THP in swap_*page_fs()") to blame?

> Signed-off-by: David Carlier <devnexen@gmail.com>
> ---
> 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);

--
Cheers,

David