[PATCH 1/2] mm/slub: preserve no-lock freeing after memcg charge failure

[Karl Mehltretter <kmehltretter@gmail.com>]

kmalloc_nolock() can obtain an object before its memcg
post-allocation charge fails.  For a single object,
memcg_slab_post_alloc_hook() rolls the allocation back through
memcg_alloc_abort_single().  That enters the regular SLUB free path,
which can take a sleeping list_lock on PREEMPT_RT even though the caller
selected a no-lock allocation.

Use kfree_nolock() when the allocation flags disallow spinning.  This
keeps the SLUB object rollback on the no-lock free path.  The failed
allocation continues to return NULL.

Fixes: af92793e52c3 ("slab: Introduce kmalloc_nolock() and kfree_nolock().")
Assisted-by: LLM
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
---
 mm/slub.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/mm/slub.c b/mm/slub.c
index 54ec125033571..a1f08338102e2 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -2519,7 +2519,10 @@ 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);
+		if (alloc_flags_allow_spinning(ac->alloc_flags))
+			memcg_alloc_abort_single(s, *p);
+		else
+			kfree_nolock(*p);
 		*p = NULL;
 	} else {
 		kmem_cache_free_bulk(s, size, p);
-- 
2.53.0