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

[Alexei Starovoitov]

On Wed, Nov 5, 2025 at 1:05 AM 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>
> ---
>  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?
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?