linux-kernel - Re: [PATCH RFC v2 13/20] slab: simplify kmalloc

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Message-ID: <CAADnVQKBt2xmqs+o0onUwd7G-0UDbE8LECnkJJUCVbywAr2tUg@mail.gmail.com>
Date: Tue, 13 Jan 2026 19:31:11 -0800
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
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>, Hao Li <hao.li@...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 <linux-mm@...ck.org>, 
	LKML <linux-kernel@...r.kernel.org>, linux-rt-devel@...ts.linux.dev, 
	bpf <bpf@...r.kernel.org>, kasan-dev <kasan-dev@...glegroups.com>
Subject: Re: [PATCH RFC v2 13/20] slab: simplify kmalloc_nolock()

On Mon, Jan 12, 2026 at 7:17 AM Vlastimil Babka <vbabka@...e.cz> wrote:
>
> The kmalloc_nolock() implementation has several complications and
> restrictions due to SLUB's cpu slab locking, lockless fastpath and
> PREEMPT_RT differences. With cpu slab usage removed, we can simplify
> things:
>
> - the local_lock_cpu_slab() macros became unused, remove them
>
> - we no longer need to set up lockdep classes on PREEMPT_RT
>
> - we no longer need to annotate ___slab_alloc as NOKPROBE_SYMBOL
>   since there's no lockless cpu freelist manipulation anymore
>
> - __slab_alloc_node() can be called from kmalloc_nolock_noprof()
>   unconditionally. It can also no longer return EBUSY. But trylock
>   failures can still happen so retry with the larger bucket if the
>   allocation fails for any reason.
>
> Note that we still need __CMPXCHG_DOUBLE, because while it was removed
> we don't use cmpxchg16b on cpu freelist anymore, we still use it on
> slab freelist, and the alternative is slab_lock() which can be
> interrupted by a nmi. Clarify the comment to mention it specifically.
>
> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>

sheaves and corresponding simplification of nolock() logic
in patches 11,12,13 look very promising to me.

Acked-by: Alexei Starovoitov <ast@...nel.org>