linux-kernel - [PATCH 2/2] mm/swap: add BUG_ON(folio==NULL) to folios_put

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Message-ID: <20250826231626.218675-2-max.kellermann@ionos.com>
Date: Wed, 27 Aug 2025 01:16:25 +0200
From: Max Kellermann <max.kellermann@...os.com>
To: akpm@...ux-foundation.org,
	david@...hat.com,
	lorenzo.stoakes@...cle.com,
	ziy@...dia.com,
	baolin.wang@...ux.alibaba.com,
	Liam.Howlett@...cle.com,
	npache@...hat.com,
	ryan.roberts@....com,
	dev.jain@....com,
	baohua@...nel.org,
	shikemeng@...weicloud.com,
	kasong@...cent.com,
	nphamcs@...il.com,
	bhe@...hat.com,
	chrisl@...nel.org,
	linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Cc: Max Kellermann <max.kellermann@...os.com>
Subject: [PATCH 2/2] mm/swap: add BUG_ON(folio==NULL) to folios_put_refs()

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@...os.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