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

[Vlastimil Babka <vbabka@suse.cz>]

On 9/12/25 12:48, Harry Yoo wrote:
> 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?

No, good catch, will fix. Thanks!

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