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

[David Carlier]

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@xxxxxxxxx>
---
 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