Re: [PATCH RFC v2 03/20] mm/slab: make caches with sheaves mergeable

[Suren Baghdasaryan <surenb@google.com>]

On Thu, Jan 15, 2026 at 11:24 PM Vlastimil Babka <vbabka@suse.cz> wrote:
>
> On 1/16/26 01:22, Suren Baghdasaryan wrote:
> > On Mon, Jan 12, 2026 at 3:17 PM Vlastimil Babka <vbabka@suse.cz> wrote:
> >>
> >> Before enabling sheaves for all caches (with automatically determined
> >> capacity), their enablement should no longer prevent merging of caches.
> >> Limit this merge prevention only to caches that were created with a
> >> specific sheaf capacity, by adding the SLAB_NO_MERGE flag to them.
> >>
> >> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> >> ---
> >>  mm/slab_common.c | 13 +++++++------
> >>  1 file changed, 7 insertions(+), 6 deletions(-)
> >>
> >> diff --git a/mm/slab_common.c b/mm/slab_common.c
> >> index 52591d9c04f3..54c17dc6d5ec 100644
> >> --- a/mm/slab_common.c
> >> +++ b/mm/slab_common.c
> >> @@ -163,9 +163,6 @@ int slab_unmergeable(struct kmem_cache *s)
> >>                 return 1;
> >>  #endif
> >>
> >> -       if (s->cpu_sheaves)
> >> -               return 1;
> >> -
> >>         /*
> >>          * We may have set a slab to be unmergeable during bootstrap.
> >>          */
> >> @@ -190,9 +187,6 @@ static struct kmem_cache *find_mergeable(unsigned int size, slab_flags_t flags,
> >>         if (IS_ENABLED(CONFIG_HARDENED_USERCOPY) && args->usersize)
> >>                 return NULL;
> >>
> >> -       if (args->sheaf_capacity)
> >> -               return NULL;
> >> -
> >>         flags = kmem_cache_flags(flags, name);
> >>
> >>         if (flags & SLAB_NEVER_MERGE)
> >> @@ -337,6 +331,13 @@ struct kmem_cache *__kmem_cache_create_args(const char *name,
> >>         flags &= ~SLAB_DEBUG_FLAGS;
> >>  #endif
> >>
> >> +       /*
> >> +        * Caches with specific capacity are special enough. It's simpler to
> >> +        * make them unmergeable.
> >> +        */
> >> +       if (args->sheaf_capacity)
> >> +               flags |= SLAB_NO_MERGE;
> >
> > So, this is very subtle and maybe not that important but the comment
> > for kmem_cache_args.sheaf_capacity claims "When slub_debug is enabled
> > for the cache, the sheaf_capacity argument is ignored.". With this
> > change this argument is not completely ignored anymore... It sets
> > SLAB_NO_MERGE even if slub_debug is enabled, doesn't it?
>
> True, but the various debug flags set by slub_debug also prevent merging so
> it doesn't change the outcome.

Yeah, I thought that would not matter much but wanted to make sure.

After finishing the review I'll have to remember to verify if that
comment on slub_debug/sheaf interplay stays true even after
args->sheaf_capacity becomes the min sheaf capacity.

>
> >> +
> >>         mutex_lock(&slab_mutex);
> >>
> >>         err = kmem_cache_sanity_check(name, object_size);
> >>
> >> --
> >> 2.52.0
> >>
>