Re: [PATCH 1/3] mm/slub: fix the race between validate_slab and slab_free

[Hyeonggon Yoo <42.hyeyoo@gmail.com>]

On Sun, May 29, 2022 at 04:15:33PM +0800, Rongwei Wang wrote:
> In use cases where allocating and freeing slab frequently, some
> error messages, such as "Left Redzone overwritten", "First byte
> 0xbb instead of 0xcc" would be printed when validating slabs.
> That's because an object has been filled with SLAB_RED_INACTIVE,
> but has not been added to slab's freelist. And between these
> two states, the behaviour of validating slab is likely to occur.
> 
> Actually, it doesn't mean the slab can not work stably. But, these
> confusing messages will disturb slab debugging more or less.
> 
> Signed-off-by: Rongwei Wang <rongwei.wang@linux.alibaba.com>

Have you observed it or it's from code inspection?

> ---
>  mm/slub.c | 40 +++++++++++++++++-----------------------
>  1 file changed, 17 insertions(+), 23 deletions(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index ed5c2c03a47a..310e56d99116 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -1374,15 +1374,12 @@ static noinline int free_debug_processing(
>  	void *head, void *tail, int bulk_cnt,
>  	unsigned long addr)
>  {
> -	struct kmem_cache_node *n = get_node(s, slab_nid(slab));
>  	void *object = head;
>  	int cnt = 0;
> -	unsigned long flags, flags2;
> +	unsigned long flags;
>  	int ret = 0;
>  
> -	spin_lock_irqsave(&n->list_lock, flags);
> -	slab_lock(slab, &flags2);
> -
> +	slab_lock(slab, &flags);
>  	if (s->flags & SLAB_CONSISTENCY_CHECKS) {
>  		if (!check_slab(s, slab))
>  			goto out;
> @@ -1414,8 +1411,7 @@ static noinline int free_debug_processing(
>  		slab_err(s, slab, "Bulk freelist count(%d) invalid(%d)\n",
>  			 bulk_cnt, cnt);
>  
> -	slab_unlock(slab, &flags2);
> -	spin_unlock_irqrestore(&n->list_lock, flags);
> +	slab_unlock(slab, &flags);
>  	if (!ret)
>  		slab_fix(s, "Object at 0x%p not freed", object);
>  	return ret;
> @@ -3304,7 +3300,7 @@ static void __slab_free(struct kmem_cache *s, struct slab *slab,
>  
>  {
>  	void *prior;
> -	int was_frozen;
> +	int was_frozen, to_take_off = 0;
>  	struct slab new;
>  	unsigned long counters;
>  	struct kmem_cache_node *n = NULL;
> @@ -3315,15 +3311,19 @@ static void __slab_free(struct kmem_cache *s, struct slab *slab,
>  	if (kfence_free(head))
>  		return;
>  
> +	n = get_node(s, slab_nid(slab));
> +	spin_lock_irqsave(&n->list_lock, flags);
> +

Oh please don't do this.

SLUB free slowpath can be hit a lot depending on workload.

__slab_free() try its best not to take n->list_lock. currently takes n->list_lock
only when the slab need to be taken from list.

Unconditionally taking n->list_lock will degrade performance.

>  	if (kmem_cache_debug(s) &&
> -	    !free_debug_processing(s, slab, head, tail, cnt, addr))
> +	    !free_debug_processing(s, slab, head, tail, cnt, addr)) {
> +
> +		spin_unlock_irqrestore(&n->list_lock, flags);
>  		return;
> +	}
>  
>  	do {
> -		if (unlikely(n)) {
> -			spin_unlock_irqrestore(&n->list_lock, flags);
> -			n = NULL;
> -		}
> +		if (unlikely(to_take_off))
> +			to_take_off = 0;
>  		prior = slab->freelist;
>  		counters = slab->counters;
>  		set_freepointer(s, tail, prior);
> @@ -3343,18 +3343,11 @@ static void __slab_free(struct kmem_cache *s, struct slab *slab,
>  				new.frozen = 1;
>  
>  			} else { /* Needs to be taken off a list */
> -
> -				n = get_node(s, slab_nid(slab));
>  				/*
> -				 * Speculatively acquire the list_lock.
>  				 * If the cmpxchg does not succeed then we may
> -				 * drop the list_lock without any processing.
> -				 *
> -				 * Otherwise the list_lock will synchronize with
> -				 * other processors updating the list of slabs.
> +				 * drop this behavior without any processing.
>  				 */
> -				spin_lock_irqsave(&n->list_lock, flags);
> -
> +				to_take_off = 1;
>  			}
>  		}
>  
> @@ -3363,8 +3356,9 @@ static void __slab_free(struct kmem_cache *s, struct slab *slab,
>  		head, new.counters,
>  		"__slab_free"));
>  
> -	if (likely(!n)) {
> +	if (likely(!to_take_off)) {
>  
> +		spin_unlock_irqrestore(&n->list_lock, flags);
>  		if (likely(was_frozen)) {
>  			/*
>  			 * The list lock was not taken therefore no list
> 
> -- 
> 2.27.0
>