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

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Message-ID: <bcfe8618-b547-49fb-97e8-e57c2fb4b7dd@suse.cz>
Date: Fri, 16 Jan 2026 12:01:23 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Harry Yoo <harry.yoo@...cle.com>
Cc: Suren Baghdasaryan <surenb@...gle.com>, Petr Tesarik <ptesarik@...e.com>,
 Christoph Lameter <cl@...two.org>, David Rientjes <rientjes@...gle.com>,
 Roman Gushchin <roman.gushchin@...ux.dev>, Hao Li <hao.li@...ux.dev>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Uladzislau Rezki <urezki@...il.com>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>,
 Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
 Alexei Starovoitov <ast@...nel.org>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, linux-rt-devel@...ts.linux.dev,
 bpf@...r.kernel.org, kasan-dev@...glegroups.com
Subject: Re: [PATCH RFC v2 03/20] mm/slab: make caches with sheaves mergeable

On 1/16/26 09:46, Harry Yoo wrote:
> On Fri, Jan 16, 2026 at 08:24:02AM +0100, Vlastimil Babka wrote:
>> On 1/16/26 01:22, Suren Baghdasaryan wrote:
>> > On Mon, Jan 12, 2026 at 3:17 PM Vlastimil Babka <vbabka@...e.cz> wrote:
>> >> @@ -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.
> 
> nit: except for slub_debug=F (SLAB_CONSISTENCY_CHECKS), since it doesn't
> prevent merging (it's in SLAB_DEBUG_FLAGS but not in SLAB_NEVER_MERGE).

Hm right. But I think that's wrong then and it should be there.
SLAB_CONSISTENCY_CHECKS is enough to stop using the fastpaths (both
before/after sheaves) so it should be a reason not to merge.