linux-kernel - Re: [PATCH v3 06/21] slab: introduce percpu sheaves bootstrap

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Message-ID: <41048e09-5dd9-42a6-b5d8-dadee3ecfd9c@suse.cz>
Date: Mon, 19 Jan 2026 10:13:51 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Harry Yoo <harry.yoo@...cle.com>, Suren Baghdasaryan <surenb@...gle.com>
Cc: 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 v3 06/21] slab: introduce percpu sheaves bootstrap

On 1/19/26 04:40, Harry Yoo wrote:
> On Sat, Jan 17, 2026 at 02:11:02AM +0000, Suren Baghdasaryan wrote:
>> On Fri, Jan 16, 2026 at 2:40 PM 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, 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.
>> 
>> I don't think kmem_cache_prefill_sheaf() handles this case, does it?
>> Or do you rely on the caller to never try prefilling a bootstrapped
>> sheaf?
> 
> If a cache doesn't have sheaves, s->sheaf_capacity should be 0,
> so the sheaf returned by kmem_cache_prefill_sheaf() should be
> "oversized" one... unless the user tries to prefill a sheaf with
> size == 0?

I'll add a

        if (unlikely(!size))
                return NULL;

to kmem_cache_prefill_sheaf() so we don't have to deal with oversized
sheaves of size 0 just for this theoretical case...