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

[Marco Elver]

On Wed, 29 Oct 2025 at 15:38, Vlastimil Babka <vbabka@xxxxxxx> wrote:
>
> 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?

Good enough - thanks.

> > The tests still pass? (CONFIG_KFENCE_KUNIT_TEST=y)
>
> They do.

Great.

Thanks,
-- Marco