Re: [PATCH 5/6] slab: validate slab before using it in alloc_single_from_partial()

[Harry Yoo <harry.yoo@oracle.com>]

On Thu, Sep 11, 2025 at 07:02:38PM +0200, Vlastimil Babka wrote:
> We touch slab->freelist and slab->inuse before checking the slab pointer
> is actually sane. Do that validation first, which will be safer. We can
> thus also remove the check from alloc_debug_processing().
> 
> This adds a new "s->flags & SLAB_CONSISTENCY_CHECKS" test but
> alloc_single_from_partial() is only called for caches with debugging
> enabled so it's acceptable.
> 
> In alloc_single_from_new_slab() we just created the struct slab and call
> alloc_debug_processing() to mainly set up redzones, tracking etc, while
> not really expecting the consistency checks to fail. Thus don't validate
> it there.
> 
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> ---
>  mm/slub.c | 17 ++++++++---------
>  1 file changed, 8 insertions(+), 9 deletions(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 909c71372a2f542b6e0d67c12ea683133b246b66..93df6e82af37c798c3fa5574c9d825f0f4a83013 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -1651,11 +1651,6 @@ static noinline bool alloc_debug_processing(struct kmem_cache *s,
>  			struct slab *slab, void *object, int orig_size)
>  {
>  	if (s->flags & SLAB_CONSISTENCY_CHECKS) {
> -		if (!validate_slab_ptr(slab)) {
> -			slab_err(s, slab, "Not a valid slab page");
> -			return false;
> -		}
> -
>  		if (!alloc_consistency_checks(s, slab, object))
>  			goto bad;
>  	}
> @@ -2825,15 +2820,19 @@ static void *alloc_single_from_partial(struct kmem_cache *s,
>  
>  	lockdep_assert_held(&n->list_lock);
>  
> +	if (s->flags & SLAB_CONSISTENCY_CHECKS) {
> +		if (!validate_slab_ptr(slab)) {
> +			slab_err(s, slab, "Not a valid slab page");
> +			return NULL;
> +		}
> +	}
> +
>  	object = slab->freelist;
>  	slab->freelist = get_freepointer(s, object);
>  	slab->inuse++;
>  
> -	if (!alloc_debug_processing(s, slab, object, orig_size)) {
> -		if (validate_slab_ptr(slab))
> -			remove_partial(n, slab);
> +	if (!alloc_debug_processing(s, slab, object, orig_size))
>  		return NULL;

Is it intentional to not remove the slab from the partial list
when alloc_debug_processing() returns false?

> -	}
>  
>  	if (slab->inuse == slab->objects) {
>  		remove_partial(n, slab);
> 
> -- 
> 2.51.0

-- 
Cheers,
Harry / Hyeonggon