[PATCH RFC 3/4] mm/slab: fix a deadlock in memcg_alloc_abort_single()

[Harry Yoo (Oracle)]

When kmalloc_nolock() successfully grabs a slab object but memcg aborts
the allocation, the object is freed via __slab_free(). Calling
__slab_free() in unknown context is not allowed and can lead to a
deadlock.

Free the object via defer_free() when spinning is not allowed.

Reported-by: Sashiko <sashiko-bot@xxxxxxxxxx>
Closes: https://sashiko.dev/#/patchset/20260610-slab_alloc_flags-v2-0-7190909db118%40kernel.org?part=9
Fixes: af92793e52c3 ("slab: Introduce kmalloc_nolock() and kfree_nolock().")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Harry Yoo (Oracle) <harry@xxxxxxxxxx>
---
 mm/slub.c | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index 85760c8ff2e2..4a3618e3967e 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -2459,7 +2459,8 @@ alloc_tagging_slab_free_hook(struct kmem_cache *s, struct slab *slab, void **p,
 
 #ifdef CONFIG_MEMCG
 
-static void memcg_alloc_abort_single(struct kmem_cache *s, void *object);
+static void memcg_alloc_abort_single(struct kmem_cache *s, void *object,
+				     bool allow_spin);
 
 static __fastpath_inline
 bool memcg_slab_post_alloc_hook(struct kmem_cache *s, gfp_t flags,
@@ -2477,7 +2478,9 @@ bool memcg_slab_post_alloc_hook(struct kmem_cache *s, gfp_t flags,
 		return true;
 
 	if (likely(size == 1)) {
-		memcg_alloc_abort_single(s, *p);
+		bool allow_spin = alloc_flags_allow_spinning(ac->alloc_flags);
+
+		memcg_alloc_abort_single(s, *p, allow_spin);
 		*p = NULL;
 	} else {
 		kmem_cache_free_bulk(s, size, p);
@@ -6436,16 +6439,20 @@ void slab_free(struct kmem_cache *s, struct slab *slab, void *object,
 #ifdef CONFIG_MEMCG
 /* Do not inline the rare memcg charging failed path into the allocation path */
 static noinline
-void memcg_alloc_abort_single(struct kmem_cache *s, void *object)
+void memcg_alloc_abort_single(struct kmem_cache *s, void *object, bool allow_spin)
 {
 	struct slab *slab = virt_to_slab(object);
 	bool init = slab_want_init_on_free(s);
-	bool allow_spin = true;
 
 	alloc_tagging_slab_free_hook(s, slab, &object, 1);
 
-	if (likely(slab_free_hook(s, object, init, false, allow_spin)))
+	if (unlikely(!slab_free_hook(s, object, init, false, allow_spin)))
+		return;
+
+	if (likely(allow_spin))
 		__slab_free(s, slab, object, object, 1, _RET_IP_);
+	else
+		defer_free(s, object);
 }
 #endif
 

-- 
2.53.0