[PATCH 6.18 170/396] mm/util: dont read __page_2 for order-1 folios in snapshot_page()

6.18-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Aboorva Devarajan <[email protected]>

commit 7441d6348c70738e9ed307510db171c7a9b3f4bf upstream.

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

If an order-1 folio is allocated at the end of a vmemmap section,
__page_2 will not exist and reading it will cause a fault.

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).

Link: https://lore.kernel.org/[email protected]
Fixes: 31a31da8a618 ("mm: move _pincount in folio to page[2] on 32bit")
Signed-off-by: Aboorva Devarajan <[email protected]>
Reported-by: Sourabh Jain <[email protected]>
Acked-by: David Hildenbrand (Arm) <[email protected]>
Reviewed-by: Lorenzo Stoakes <[email protected]>
Reviewed-by: Matthew Wilcox (Oracle) <[email protected]>
Reviewed-by: Luiz Capitulino <[email protected]>
Cc: Liam R. Howlett <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Mike Rapoport <[email protected]>
Cc: "Ritesh Harjani (IBM)" <[email protected]>
Cc: Suren Baghdasaryan <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: <[email protected]> # v6.15+
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
---
 mm/util.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/mm/util.c
+++ b/mm/util.c
@@ -1263,7 +1263,7 @@ again:
 	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);