[PATCH v5] slab: reset obj_ext when it is not actually valid during freeing

[Hao Ge <hao.ge@linux.dev>]

From: Hao Ge <gehao@kylinos.cn>

If obj_exts allocation failed, slab->obj_exts is set to OBJEXTS_ALLOC_FAIL,
But we did not clear it when freeing the slab. Since OBJEXTS_ALLOC_FAIL and
MEMCG_DATA_OBJEXTS currently share the same bit position, during the
release of the associated folio, a VM_BUG_ON_FOLIO() check in
folio_memcg_kmem() is triggered because it was mistakenly assumed that
a valid folio->memcg_data was not cleared before freeing the folio.

When freeing a slab, we clear slab->obj_exts and reset it to 0
if the obj_ext array has been successfully allocated.
So let's reset slab->obj_exts to 0 when freeing a slab if
the obj_ext array allocated fail to allow them to be returned
to the buddy system more smoothly.

Signed-off-by: Hao Ge <gehao@kylinos.cn>
---
v5: Adopt the simpler solution proposed by Vlastimil;
    Many thanks to him
---
 mm/slub.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/mm/slub.c b/mm/slub.c
index b1f15598fbfd..2e4340c75be2 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -2170,8 +2170,16 @@ static inline void free_slab_obj_exts(struct slab *slab)
 	struct slabobj_ext *obj_exts;
 
 	obj_exts = slab_obj_exts(slab);
-	if (!obj_exts)
+	if (!obj_exts) {
+		/*
+		 * If obj_exts allocation failed, slab->obj_exts is set to OBJEXTS_ALLOC_FAIL,
+		 * In this case, we will end up here.
+		 * Therefore, we should clear the OBJEXTS_ALLOC_FAIL flag first when freeing a slab.
+		 * Then let's set it to 0 as below.
+		 */
+		slab->obj_exts = 0;
 		return;
+	}
 
 	/*
 	 * obj_exts was created with __GFP_NO_OBJ_EXT flag, therefore its
-- 
2.25.1