linux-kernel - Re: [PATCH v3 14/21] slab: simplify kmalloc

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Message-ID: <aXGC_JRmz3ICjMHW@hyeyoo>
Date: Thu, 22 Jan 2026 10:53:00 +0900
From: Harry Yoo <harry.yoo@...cle.com>
To: Vlastimil Babka <vbabka@...e.cz>
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>,
        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 v3 14/21] slab: simplify kmalloc_nolock()

On Fri, Jan 16, 2026 at 03:40:34PM +0100, Vlastimil Babka 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:
> 
> - relax the PREEMPT_RT context checks as they were before commit
>   a4ae75d1b6a2 ("slab: fix kmalloc_nolock() context check for
>   PREEMPT_RT") and also reference the explanation comment in the page
>   allocator
> 
> - 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>
> ---

What a nice cleanup!

Looks good to me,
Reviewed-by: Harry Yoo <harry.yoo@...cle.com>

with a nit below.

>  mm/slab.h |   1 -
>  mm/slub.c | 144 +++++++++++++-------------------------------------------------
>  2 files changed, 29 insertions(+), 116 deletions(-)
> 
> diff --git a/mm/slab.h b/mm/slab.h
> index 4efec41b6445..e9a0738133ed 100644
> --- a/mm/slab.h
> +++ b/mm/slab.h
> @@ -5268,10 +5196,11 @@ void *kmalloc_nolock_noprof(size_t size, gfp_t gfp_flags, int node)
>  	if (!(s->flags & __CMPXCHG_DOUBLE) && !kmem_cache_debug(s))
>  		/*
>  		 * kmalloc_nolock() is not supported on architectures that
> -		 * don't implement cmpxchg16b, but debug caches don't use
> -		 * per-cpu slab and per-cpu partial slabs. They rely on
> -		 * kmem_cache_node->list_lock, so kmalloc_nolock() can
> -		 * attempt to allocate from debug caches by
> +		 * don't implement cmpxchg16b and thus need slab_lock()
> +		 * which could be preempted by a nmi.

nit: I think now this limitation can be removed because the only slab
lock used in the allocation path is get_partial_node() ->
__slab_update_freelist(), but it is always used under n->list_lock.

Being preempted by a NMI while holding the slab lock is fine because
NMI context should fail to acquire n->list_lock and bail out.

But no hurry on this, it's probably not important enough to delay
this series :)

> +		 * But debug caches don't use that and only rely on
> +		 * kmem_cache_node->list_lock, so kmalloc_nolock() can attempt
> +		 * to allocate from debug caches by
>  		 * spin_trylock_irqsave(&n->list_lock, ...)
>  		 */
>  		return NULL;
>

-- 
Cheers,
Harry / Hyeonggon