linux-kernel - Re: [PATCH v4 07/22] slab: introduce percpu sheaves bootstrap

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <qrekwm7js5t4kmahu3toqnrepnvk7ve5h624f6hm262mmybvtx@rewwd4rbvf3b>
Date: Mon, 26 Jan 2026 14:13:51 +0800
From: Hao Li <hao.li@...ux.dev>
To: Vlastimil Babka <vbabka@...e.cz>
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>, Andrew Morton <akpm@...ux-foundation.org>, 
	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 v4 07/22] slab: introduce percpu sheaves bootstrap

On Fri, Jan 23, 2026 at 07:52:45AM +0100, Vlastimil Babka 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.
> Thus sharing the single bootstrap sheaf like this for multiple caches
> and cpus is safe.
> 
> Reviewed-by: Harry Yoo <harry.yoo@...cle.com>
> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>
> ---
>  mm/slab.h        |  12 ++++++
>  mm/slab_common.c |   2 +-
>  mm/slub.c        | 123 ++++++++++++++++++++++++++++++++++++-------------------
>  3 files changed, 95 insertions(+), 42 deletions(-)

Tiny consistency nit: in kfree_rcu_sheaf(), there's a remaining "if
(s->cpu_sheaves)" that could be replaced with "if (cache_has_sheaves(s))" for
consistency. It's trivial, so no need to respin - happy to have it addressed
opportunistically.

The rest looks great to me!

Reviewed-by: Hao Li <hao.li@...ux.dev>

-- 
Thanks,
Hao