[PATCH v2] slab: Fix obj_ext is mistakenly considered NULL due to race condition

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

From: Hao Ge <gehao@kylinos.cn>

If two competing threads enter alloc_slab_obj_exts(), if the process
that allocates the vector wins cmpxchg(), and the other thread mistakenly
assume slab->obj_ext is still empty due to its own allocation failure. This
will then trigger warnings enforced by CONFIG_MEM_ALLOC_PROFILING_DEBUG
checks in the subsequent free path.

Therefore, let's add an additional check when the process that allocates
the vector loses the cmpxchg()

Suggested-by: Harry Yoo <harry.yoo@oracle.com>
Signed-off-by: Hao Ge <gehao@kylinos.cn>
---
v2: Revise the solution according to Harry's suggestion.
    Add Suggested-by: Harry Yoo <harry.yoo@oracle.com>
---
 mm/slub.c | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index d4403341c9df..d7bfec6c0171 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -2052,9 +2052,9 @@ static inline void mark_objexts_empty(struct slabobj_ext *obj_exts)
 	}
 }
 
-static inline void mark_failed_objexts_alloc(struct slab *slab)
+static inline bool mark_failed_objexts_alloc(struct slab *slab)
 {
-	cmpxchg(&slab->obj_exts, 0, OBJEXTS_ALLOC_FAIL);
+	return cmpxchg(&slab->obj_exts, 0, OBJEXTS_ALLOC_FAIL) == 0;
 }
 
 static inline void handle_failed_objexts_alloc(unsigned long obj_exts,
@@ -2076,7 +2076,7 @@ static inline void handle_failed_objexts_alloc(unsigned long obj_exts,
 #else /* CONFIG_MEM_ALLOC_PROFILING_DEBUG */
 
 static inline void mark_objexts_empty(struct slabobj_ext *obj_exts) {}
-static inline void mark_failed_objexts_alloc(struct slab *slab) {}
+static inline bool mark_failed_objexts_alloc(struct slab *slab) { return false; }
 static inline void handle_failed_objexts_alloc(unsigned long obj_exts,
 			struct slabobj_ext *vec, unsigned int objects) {}
 
@@ -2124,8 +2124,14 @@ int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s,
 				   slab_nid(slab));
 	}
 	if (!vec) {
-		/* Mark vectors which failed to allocate */
-		mark_failed_objexts_alloc(slab);
+		/*
+		 * Try to mark vectors which failed to allocate
+		 * If this operation fails, there may be a racing process
+		 * that has already completed the allocation.
+		 */
+		if (!mark_failed_objexts_alloc(slab) &&
+		    slab_obj_exts(slab))
+			return 0;
 
 		return -ENOMEM;
 	}
-- 
2.25.1