Re: [PATCH v4 07/22] slab: introduce percpu sheaves bootstrap

Hao Li
2026-01-26T06:14:08Z

On Fri, Jan 23, 2026 at 07:52:45AM +0100, Vlastimil Babka wrote:
> Until now, kmem_cache->cpu_sheaves was !NULL only for caches with
> sheaves enabled. Since we want to enable them for almost all caches,
> it's suboptimal to test the pointer in the fast paths, so instead
> allocate it for all caches in do_kmem_cache_create(). Instead of testing
> the cpu_sheaves pointer to recognize caches (yet) without sheaves, test
> kmem_cache->sheaf_capacity for being 0, where needed, using a new
> cache_has_sheaves() helper.
>
> However, for the fast paths sake we also assume that the main sheaf
> always exists (pcs->main is !NULL), and during bootstrap we cannot
> allocate sheaves yet.
>
> Solve this by introducing a single static bootstrap_sheaf that's
> assigned as pcs->main during bootstrap. It has a size of 0, so during
> allocations, the fast path will find it's empty. Since the size of 0
> matches sheaf_capacity of 0, the freeing fast paths will find it's
> "full". In the slow path handlers, we use cache_has_sheaves() to
> recognize that the cache doesn't (yet) have real sheaves, and fall back.
> Thus sharing the single bootstrap sheaf like this for multiple caches
> and cpus is safe.
>
> Reviewed-by: Harry Yoo <harry.yoo@oracle.com>
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> ---
> mm/slab.h | 12 ++++++
> mm/slab_common.c | 2 +-
> mm/slub.c | 123 ++++++++++++++++++++++++++++++++++++-------------------
> 3 files changed, 95 insertions(+), 42 deletions(-)

Tiny consistency nit: in kfree_rcu_sheaf(), there's a remaining "if
(s->cpu_sheaves)" that could be replaced with "if (cache_has_sheaves(s))" for
consistency. It's trivial, so no need to respin - happy to have it addressed
opportunistically.

The rest looks great to me!

Reviewed-by: Hao Li <hao.li@linux.dev>

--
Thanks,
Hao