Re: [PATCH v2 01/13] mm/slab: skip kfence objects in allocation profiling

[Vlastimil Babka (SUSE)]

On 7/22/26 14:47, Hao Li wrote:
> On Mon, Jul 20, 2026 at 04:16:15PM +0200, Vlastimil Babka (SUSE) wrote:
>> struct kfence_metadata only contains struct slabobj_ext with
>> CONFIG_MEMCG, which is then used for the "fake" slab's obj_exts field.
>> If CONFIG_MEMCG is enabled, the struct can also end up used for memory
>> allocation profiling. If CONFIG_MEMCG is disabled but profiling is
>> enabled, it will end up allocating its obj_exts via
>> prepare_slab_obj_exts_hook() and assigning them to the fake struct slab.
>> These will probably then never be freed.
>> 
>> So things sorta work, but not always in the intended and optimal way.
>> The upcoming changes to slabobj_ext layout would additionally need a
>> proper refactoring to keep working.
>> 
>> However, there's little benefit in accounting KFENCE objects. KFENCE
>> allocations are rare and there can be only CONFIG_KFENCE_NUM_OBJECTS
>> (default to 255) outstanding ones at any time. For any callsite
>> prominent enough in the memory allocation profiling stats, allocations
>> served from KFENCE will be lost in the noise.
>> 
>> Thus let's not complicate things and simply stop accounting KFENCE
>> objects in allocation profiling and skip them in the related slab hooks.
>> 
>> We also need to skip kfence objects in mark_obj_codetag_empty() in case
>> a sheaf is allocated from kfence, per earlier sashiko review.
>> 
>> Signed-off-by: Vlastimil Babka (SUSE) <vbabka@xxxxxxxxxx>
>> ---
>>  Documentation/mm/allocation-profiling.rst |  7 +++++++
>>  mm/slub.c                                 | 11 +++++++++++
>>  2 files changed, 18 insertions(+)
>> 
>> diff --git a/Documentation/mm/allocation-profiling.rst b/Documentation/mm/allocation-profiling.rst
>> index 5389d241176a..d02eb54ee8f2 100644
>> --- a/Documentation/mm/allocation-profiling.rst
>> +++ b/Documentation/mm/allocation-profiling.rst
>> @@ -112,3 +112,10 @@ break it out by rhashtable type.
>>  
>>  - Then, use the following form for your allocations:
>>    alloc_hooks_tag(ht->your_saved_tag, kmalloc_noprof(...))
>> +
>> +Notes
>> +=====
>> +
>> +- When a slab object is allocated from KFENCE, its accounting is skipped.
>> +  KFENCE allocations are rare and limited to a small number, so this omission
>> +  is negligible.
>> diff --git a/mm/slub.c b/mm/slub.c
>> index 0337e60db5ac..76acb78f2655 100644
>> --- a/mm/slub.c
>> +++ b/mm/slub.c
>> @@ -2076,6 +2076,11 @@ static inline void mark_obj_codetag_empty(const void *obj)
>>  		struct slabobj_ext *ext = slab_obj_ext(obj_slab,
>>  						       slab_exts, offs);
>>  
>> +		if (is_kfence_address(obj)) {
>> +			put_slab_obj_exts(slab_exts);
>> +			return;
>> +		}
>> +
>>  		if (unlikely(is_codetag_empty(&ext->ref))) {
>>  			put_slab_obj_exts(slab_exts);
>>  			return;
>> @@ -2352,6 +2357,9 @@ __alloc_tagging_slab_alloc_hook(struct kmem_cache *s, void *object, gfp_t flags,
>>  	if (alloc_flags & SLAB_ALLOC_NO_RECURSE)
>>  		return;
>>  
>> +	if (is_kfence_address(object))
>> +		return;
>> +
>>  	slab = virt_to_slab(object);
>>  	obj_exts = prepare_slab_obj_exts_hook(s, slab, flags, alloc_flags, object);
>>  	/*
>> @@ -2399,6 +2407,9 @@ __alloc_tagging_slab_free_hook(struct kmem_cache *s, struct slab *slab, void **p
>>  	for (i = 0; i < objects; i++) {
>>  		unsigned int off = obj_to_index(s, slab, p[i]);
> 
> I noticed obj_to_index also does the `if (is_kfence_address(obj))` check, so
> maybe we could use:
> 
> if (is_kfence_address(p[i]))
> 	continue;
> off = __obj_to_index(cache, slab_address(slab), obj);

Hm it would work, but go away after patch 5/13 anyway, so will leave as is.
Thanks!

>>  
>> +		if (is_kfence_address(p[i]))
>> +			continue;
>> +
>>  		alloc_tag_sub(&slab_obj_ext(slab, obj_exts, off)->ref, s->size);
>>  	}
>>  	put_slab_obj_exts(obj_exts);
>> 
>> -- 
>> 2.55.0
>> 
>