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

[Harry Yoo]

On Thu, Oct 01, 2026 at 06:40:55AM +0200, Karl Mehltretter wrote:
> 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.

When this happens, the object is not charged by memcg.

The kernel should not invoke memcg_slab_free_hook() (called by
kfree_nolock()) for a slab object that is not charged by memcg.

> Fixes: af92793e52c3 ("slab: Introduce kmalloc_nolock() and kfree_nolock().")
> Assisted-by: LLM
> Signed-off-by: Karl Mehltretter <kmehltretter@xxxxxxxxx>
> ---
>  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

-- 
Cheers,
Harry / Hyeonggon