linux-kernel - [PATCH v3] slab: Add check for memcg_data != OBJEXTS_ALLOC_FAIL in folio_memcg

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Message-Id: <20251014152751.499376-1-hao.ge@linux.dev>
Date: Tue, 14 Oct 2025 23:27:51 +0800
From: Hao Ge <hao.ge@...ux.dev>
To: Vlastimil Babka <vbabka@...e.cz>,
	Alexei Starovoitov <ast@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Shakeel Butt <shakeel.butt@...ux.dev>,
	Michal Hocko <mhocko@...nel.org>,
	Roman Gushchin <roman.gushchin@...ux.dev>,
	Muchun Song <muchun.song@...ux.dev>,
	Suren Baghdasaryan <surenb@...gle.com>
Cc: Harry Yoo <harry.yoo@...cle.com>,
	cgroups@...r.kernel.org,
	linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Hao Ge <gehao@...inos.cn>
Subject: [PATCH v3] slab: Add check for memcg_data != OBJEXTS_ALLOC_FAIL in folio_memcg_kmem

From: Hao Ge <gehao@...inos.cn>

Since OBJEXTS_ALLOC_FAIL and MEMCG_DATA_OBJEXTS currently share
the same bit position, we cannot determine whether memcg_data still
points to the slabobj_ext vector simply by checking
folio->memcg_data & MEMCG_DATA_OBJEXTS.

If obj_exts allocation failed, slab->obj_exts is set to OBJEXTS_ALLOC_FAIL,
and during the release of the associated folio, the BUG check is triggered
because it was mistakenly assumed that a valid folio->memcg_data
was not cleared before freeing the folio.

So let's check for memcg_data != OBJEXTS_ALLOC_FAIL in folio_memcg_kmem.

Fixes: 7612833192d5 ("slab: Reuse first bit for OBJEXTS_ALLOC_FAIL")
Suggested-by: Harry Yoo <harry.yoo@...cle.com>
Signed-off-by: Hao Ge <gehao@...inos.cn>
---
v3: Simplify the solution, per Harry's suggestion in the v1 comments
    Add Suggested-by: Harry Yoo <harry.yoo@...cle.com>
---
 include/linux/memcontrol.h | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 873e510d6f8d..7ed15f858dc4 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -534,7 +534,9 @@ static inline struct mem_cgroup *get_mem_cgroup_from_objcg(struct obj_cgroup *ob
 static inline bool folio_memcg_kmem(struct folio *folio)
 {
 	VM_BUG_ON_PGFLAGS(PageTail(&folio->page), &folio->page);
-	VM_BUG_ON_FOLIO(folio->memcg_data & MEMCG_DATA_OBJEXTS, folio);
+	VM_BUG_ON_FOLIO((folio->memcg_data != OBJEXTS_ALLOC_FAIL) &&
+			(folio->memcg_data & MEMCG_DATA_OBJEXTS),
+			folio);
 	return folio->memcg_data & MEMCG_DATA_KMEM;
 }
 
-- 
2.25.1