Re: [PATCH RFC 01/19] slab: move kfence_alloc() out of internal bulk alloc

[Vlastimil Babka]

On 10/23/25 17:20, Marco Elver wrote:
> On Thu, 23 Oct 2025 at 15:53, Vlastimil Babka <vbabka@xxxxxxx> 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@xxxxxxxxxx>
>> Cc: Marco Elver <elver@xxxxxxxxxx>
>> Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
>> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
>> ---
>> @@ -7457,6 +7458,20 @@ int kmem_cache_alloc_bulk_noprof(struct kmem_cache *s, gfp_t flags, size_t size,
>>         if (unlikely(!s))
>>                 return 0;
>>
>> +       /*
>> +        * to make things simpler, only assume at most once kfence allocated
>> +        * object per bulk allocation and choose its index randomly
>> +        */

Here's a comment...

>> +       kfence_obj = kfence_alloc(s, s->object_size, flags);
>> +
>> +       if (unlikely(kfence_obj)) {
>> +               if (unlikely(size == 1)) {
>> +                       p[0] = kfence_obj;
>> +                       goto out;
>> +               }
>> +               size--;
>> +       }
>> +
>>         if (s->cpu_sheaves)
>>                 i = alloc_from_pcs_bulk(s, size, p);
>>
>> @@ -7468,10 +7483,23 @@ int kmem_cache_alloc_bulk_noprof(struct kmem_cache *s, gfp_t flags, size_t size,
>>                 if (unlikely(__kmem_cache_alloc_bulk(s, flags, size - i, p + i) == 0)) {
>>                         if (i > 0)
>>                                 __kmem_cache_free_bulk(s, i, p);
>> +                       if (kfence_obj)
>> +                               __kfence_free(kfence_obj);
>>                         return 0;
>>                 }
>>         }
>>
>> +       if (unlikely(kfence_obj)) {
> 
> Might be nice to briefly write a comment here in code as well instead
> of having to dig through the commit logs.

... is the one above enough? The commit log doesn't have much more on this
aspect. Or what would you add?

> The tests still pass? (CONFIG_KFENCE_KUNIT_TEST=y)

They do.

Thanks,
Vlastimil

>> +               int idx = get_random_u32_below(size + 1);
>> +
>> +               if (idx != size)
>> +                       p[size] = p[idx];
>> +               p[idx] = kfence_obj;
>> +
>> +               size++;
>> +       }
>> +
>> +out:
>>         /*
>>          * memcg and kmem_cache debug support and memory initialization.
>>          * Done outside of the IRQ disabled fastpath loop.
>>
>> --
>> 2.51.1