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

[Hao Li <hao.li@linux.dev>]

On Fri, Jul 24, 2026 at 05:58:36PM +0200, Vlastimil Babka (SUSE) wrote:
> 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@kernel.org>
> >> ---
> >>  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.

make sense!

> 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
> >> 
> > 
> 

-- 
Thanks,
Hao