Re: [PATCH 2/5] slab: move kfence_alloc() out of internal bulk alloc

[Vlastimil Babka <vbabka@suse.cz>]

On 11/6/25 03:39, Alexei Starovoitov wrote:
> On Wed, Nov 5, 2025 at 1:05 AM Vlastimil Babka <vbabka@suse.cz> wrote:
>>
>> SLUB's internal bulk allocation __kmem_cache_alloc_bulk() can currently
>> allocate some objects from KFENCE, i.e. when refilling a sheaf. It works
>> but it's conceptually the wrong layer, as KFENCE allocations should only
>> happen when objects are actually handed out from slab to its users.
>>
>> Currently for sheaf-enabled caches, slab_alloc_node() can return KFENCE
>> object via kfence_alloc(), but also via alloc_from_pcs() when a sheaf
>> was refilled with KFENCE objects. Continuing like this would also
>> complicate the upcoming sheaf refill changes.
>>
>> Thus remove KFENCE allocation from __kmem_cache_alloc_bulk() and move it
>> to the places that return slab objects to users. slab_alloc_node() is
>> already covered (see above). Add kfence_alloc() to
>> kmem_cache_alloc_from_sheaf() to handle KFENCE allocations from
>> prefilled sheafs, with a comment that the caller should not expect the
>> sheaf size to decrease after every allocation because of this
>> possibility.
>>
>> For kmem_cache_alloc_bulk() implement a different strategy to handle
>> KFENCE upfront and rely on internal batched operations afterwards.
>> Assume there will be at most once KFENCE allocation per bulk allocation
>> and then assign its index in the array of objects randomly.
>>
>> Cc: Alexander Potapenko <glider@google.com>
>> Cc: Marco Elver <elver@google.com>
>> Cc: Dmitry Vyukov <dvyukov@google.com>
>> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
>> ---
>>  mm/slub.c | 44 ++++++++++++++++++++++++++++++++++++--------
>>  1 file changed, 36 insertions(+), 8 deletions(-)
>>
>> diff --git a/mm/slub.c b/mm/slub.c
>> index 074abe8e79f8..0237a329d4e5 100644
>> --- a/mm/slub.c
>> +++ b/mm/slub.c
>> @@ -5540,6 +5540,9 @@ int kmem_cache_refill_sheaf(struct kmem_cache *s, gfp_t gfp,
>>   *
>>   * The gfp parameter is meant only to specify __GFP_ZERO or __GFP_ACCOUNT
>>   * memcg charging is forced over limit if necessary, to avoid failure.
>> + *
>> + * It is possible that the allocation comes from kfence and then the sheaf
>> + * size is not decreased.
>>   */
>>  void *
>>  kmem_cache_alloc_from_sheaf_noprof(struct kmem_cache *s, gfp_t gfp,
>> @@ -5551,7 +5554,10 @@ kmem_cache_alloc_from_sheaf_noprof(struct kmem_cache *s, gfp_t gfp,
>>         if (sheaf->size == 0)
>>                 goto out;
>>
>> -       ret = sheaf->objects[--sheaf->size];
>> +       ret = kfence_alloc(s, s->object_size, gfp);
>> +
>> +       if (likely(!ret))
>> +               ret = sheaf->objects[--sheaf->size];
> 
> Judging by this direction you plan to add it to kmalloc/alloc_from_pcs too?

No, kmem_cache_alloc_from_sheaf() is a new API for use cases like maple
tree, it's different from the internal alloc_from_pcs() caching.

> If so it will break sheaves+kmalloc_nolock approach in
> your prior patch set, since kfence_alloc() is not trylock-ed.
> Or this will stay kmem_cache specific?

I rechecked the result of the full RFC and kfence_alloc() didn't appear in
kmalloc_nolock() path. I would say this patch moved it rather in the
opposite direction, away from internal layers that could end up in
kmalloc_nolock() path when kmalloc caches have sheaves.