[PATCH v2] mm/util: don't read __page_2 for order-1 folios in snapshot_page()

[Aboorva Devarajan <aboorvad@linux.ibm.com>]

snapshot_page() currently reads __page_2 after checking nr_pages > 1,
but it should only do so when nr_pages > 2.

During DLPAR memory remove on a 22 TB ppc64le LPAR, snapshot_page()
oopsed on the page isolation path while reading an order-1 folio's
__page_2 from an adjacent absent section (unmapped vmemmap).

Fix this to avoid reading memmap that doesn't exist (e.g., a vmemmap
hole).

Fixes: 31a31da8a618 ("mm: move _pincount in folio to page[2] on 32bit")
Cc: stable@vger.kernel.org # v6.15+
Reported-by: Sourabh Jain <sourabhjain@linux.ibm.com>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Lorenzo Stoakes <ljs@kernel.org>
Signed-off-by: Aboorva Devarajan <aboorvad@linux.ibm.com>
---
v1 -> v2:
 - Condense the commit message.
 - Drop the code comment.

 mm/util.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/util.c b/mm/util.c
index af2c2103f0d95..34cb43b3eaa4c 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -1353,7 +1353,7 @@ void snapshot_page(struct page_snapshot *ps, const struct page *page)
 	if (ps->idx < MAX_FOLIO_NR_PAGES) {
 		memcpy(&ps->folio_snapshot, foliop, 2 * sizeof(struct page));
 		nr_pages = folio_nr_pages(&ps->folio_snapshot);
-		if (nr_pages > 1)
+		if (nr_pages > 2)
 			memcpy(&ps->folio_snapshot.__page_2, &foliop->__page_2,
 			       sizeof(struct page));
 		set_ps_flags(ps, foliop, page);
-- 
2.54.0