mm/page_io: use sio->len for PSWPIN accounting in sio_read_complete()

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.

While large folios cannot currently reach this path (SWP_FS_OPS and
SWP_SYNCHRONOUS_IO are mutually exclusive, and mTHP swap-in allocation is
gated on SWP_SYNCHRONOUS_IO), the accounting is semantically inconsistent
with the per-memcg path which correctly uses 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).

Link: https://lore.kernel.org/20260402061408.36119-1-devnexen@gmail.com
Signed-off-by: David Carlier <devnexen@gmail.com>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Cc: Baoquan He <bhe@redhat.com>
Cc: Chris Li <chrisl@kernel.org>
Cc: Kairui Song <kasong@tencent.com>
Cc: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: NeilBrown <neil@brown.name>
Cc: Nhat Pham <nphamcs@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/page_io.c b/mm/page_io.c
index 93d03d9e2a6a7e9d748ca0aa5112edb201f72f03..70cea9e24d2fd3fd9bd2763d19dc10985497f88c 100644 (file)
--- 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);