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

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Message-ID: <4e73da60-b58d-40bd-86ed-a0243967017b@suse.cz>
Date: Fri, 16 Jan 2026 08:24:02 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Suren Baghdasaryan <surenb@...gle.com>
Cc: Harry Yoo <harry.yoo@...cle.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 01:22, Suren Baghdasaryan wrote:
> On Mon, Jan 12, 2026 at 3:17 PM Vlastimil Babka <vbabka@...e.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@...e.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.

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