Re: [PATCH RFC 01/12] mm/slab: skip kfence objects in allocation profiling

[Vlastimil Babka (SUSE)]

On 7/15/26 18:02, Suren Baghdasaryan wrote:
> On Wed, Jul 15, 2026 at 3:10 AM Vlastimil Babka (SUSE)
> <vbabka@xxxxxxxxxx> wrote:
>>
>> struct kfence_metadata only has obj_exts with CONFIG_MEMCG.
> 
> I don't quite understand this statement. obj_exts are allocated when
> either CONFIG_MEMCG or CONFIG_MEM_ALLOC_PROFILING is enabled.

See in mm/kfence/kfence.h

struct kfence_metadata {
...
#ifdef CONFIG_MEMCG
        struct slabobj_ext obj_exts;
#endif
...

then in mm/kfence/core.c kfence_init_pool()

#ifdef CONFIG_MEMCG
                struct slab *slab = page_slab(page);
                slab->obj_exts = (unsigned long)&kfence_metadata_init[i / 2 - 1].obj_exts |
                                 MEMCG_DATA_OBJEXTS;
#endif

So the slab kfence fakes only has this pre-inited obj_exts
with CONFIG_MEMCG.

What happens if we run memalloc profiling without MEMCG and this
fake slab doesn't have a pre-assigned obj_exts? I guess
__alloc_tagging_slab_alloc_hook() will allocate it via
prepare_slab_obj_exts_hook().

But it's something that was done consciously and probably
just accidentally works.

>> If it's
>> enabled, it does also work for allocation profiling, but there's little
>> value recording tags for KFENCE objects.
> 
> Unless we are leaking them, right?

Well if there are leaks in a particular callsite, we should see that from all
the allocations that don't end up in kfence (as kfence allocations are rare).
So we are very unlikely to miss a leak due to this.

>> Furthermore it would complicate
>> the upcoming changes, so just skip them in the slab hooks.
> 
> Ok, I can understand that. If we do this, we should document that
> kfence objects are no longer tracked.
> 
>>
>> Signed-off-by: Vlastimil Babka (SUSE) <vbabka@xxxxxxxxxx>
>> ---
>>  mm/slub.c | 6 ++++++
>>  1 file changed, 6 insertions(+)
>>
>> diff --git a/mm/slub.c b/mm/slub.c
>> index 0337e60db5ac..a4be70d080fb 100644
>> --- a/mm/slub.c
>> +++ b/mm/slub.c
>> @@ -2352,6 +2352,9 @@ __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 +2402,9 @@ __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]);
>>
>> +               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
>>