linux-kernel - Re: [PATCH RFC 06/19] slab: introduce percpu sheaves bootstrap

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Message-ID: <48e0bc5b-ccee-4fce-8a89-a32f79228bb6@suse.cz>
Date: Wed, 29 Oct 2025 16:51:02 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Chris Mason <clm@...a.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
 Christoph Lameter <cl@...two.org>, David Rientjes <rientjes@...gle.com>,
 Roman Gushchin <roman.gushchin@...ux.dev>, Harry Yoo <harry.yoo@...cle.com>,
 Uladzislau Rezki <urezki@...il.com>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>,
 Suren Baghdasaryan <surenb@...gle.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 06/19] slab: introduce percpu sheaves bootstrap

On 10/24/25 17:29, Chris Mason wrote:
> On Thu, 23 Oct 2025 15:52:28 +0200 Vlastimil Babka <vbabka@...e.cz> 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.
>> 
>> 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 check sheaf_capacity 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.
>>
> 
> Hi Vlastimil,
> 
> We're trying out the AI patch review automation on the BPF commits and it had
> some questions about a few of these.  Since the goal is to actually test the
> automation, I'm putting them in unedited, but I did try and make sure they
> were valid before sending.
> 
>> diff --git a/mm/slub.c b/mm/slub.c
>> index a6e58d3708f4..ecb10ed5acfe 100644
>> --- a/mm/slub.c
>> +++ b/mm/slub.c
> 
> [ ... ]
> 
>> @@ -7733,8 +7782,7 @@ static void free_kmem_cache_nodes(struct kmem_cache *s)
>>  void __kmem_cache_release(struct kmem_cache *s)
>>  {
>>  	cache_random_seq_destroy(s);
>> -	if (s->cpu_sheaves)
>> -		pcs_destroy(s);
>> +	pcs_destroy(s);
> 
> Can pcs_destroy() handle a NULL cpu_sheaves pointer? Looking at the
> error path in do_kmem_cache_create():

Good catch! I will leave the condition in __kmem_cache_release().
Thanks!