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

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/20260708201954.686111-1-aboorvad@linux.ibm.com
Fixes: 31a31da8a618 ("mm: move _pincount in folio to page[2] on 32bit")
Signed-off-by: Aboorva Devarajan <aboorvad@linux.ibm.com>
Reported-by: Sourabh Jain <sourabhjain@linux.ibm.com>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Lorenzo Stoakes <ljs@kernel.org>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Luiz Capitulino <luizcap@redhat.com>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: "Ritesh Harjani (IBM)" <ritesh.list@gmail.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: <stable@vger.kernel.org> # v6.15+
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/util.c b/mm/util.c
index af2c2103f0d952181d6491fb34122d457c174026..34cb43b3eaa4c3e9fb5074b31fb5553064c0614c 100644 (file)
--- a/mm/util.c
+++ b/mm/util.c
@@ -1353,7 +1353,7 @@ 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);