[PATCH 2/2] mm/swap: add BUG_ON(folio==NULL) to folios_put_refs()

[Max Kellermann <max.kellermann@ionos.com>]

It is not legal to have NULL pointers in a folio_batch.

However, the Ceph code does exactly this, and a refactoring patch gone
wrong has exposed this to folios_put_refs(), see
https://lore.kernel.org/ceph-devel/aK4v548CId5GIKG1@swift.blarg.de/

I believe this should Oops instead of crashing due to NULL pointer
reference (guarded by is_huge_zero_folio(), which may silently hide
the bug).

Signed-off-by: Max Kellermann <max.kellermann@ionos.com>
---
 mm/swap.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/mm/swap.c b/mm/swap.c
index 3632dd061beb..07ccda00e7ee 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -952,6 +952,7 @@ void folios_put_refs(struct folio_batch *folios, unsigned int *refs)
 	for (i = 0, j = 0; i < folios->nr; i++) {
 		struct folio *folio = folios->folios[i];
 		unsigned int nr_refs = refs ? refs[i] : 1;
+		BUG_ON(folio == NULL);
 
 		if (is_huge_zero_folio(folio))
 			continue;
-- 
2.47.2