linux-kernel - Re: [PATCH v3 07/21] slab: make percpu sheaves compatible with kmalloc_nolock()/kfree

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Message-ID: <008029ff-3fd8-49cf-8aa7-71b98dc15be9@suse.cz>
Date: Mon, 19 Jan 2026 11:23:04 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Harry Yoo <harry.yoo@...cle.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>,
 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 07/21] slab: make percpu sheaves compatible with
 kmalloc_nolock()/kfree_nolock()

On 1/19/26 11:09, Vlastimil Babka wrote:
> On 1/19/26 05:31, Harry Yoo wrote:
>> On Fri, Jan 16, 2026 at 03:40:27PM +0100, Vlastimil Babka wrote:
>>> Before we enable percpu sheaves for kmalloc caches, we need to make sure
>>> kmalloc_nolock() and kfree_nolock() will continue working properly and
>>> not spin when not allowed to.
>>> 
>>> Percpu sheaves themselves use local_trylock() so they are already
>>> compatible. We just need to be careful with the barn->lock spin_lock.
>>> Pass a new allow_spin parameter where necessary to use
>>> spin_trylock_irqsave().
>>> 
>>> In kmalloc_nolock_noprof() we can now attempt alloc_from_pcs() safely,
>>> for now it will always fail until we enable sheaves for kmalloc caches
>>> next. Similarly in kfree_nolock() we can attempt free_to_pcs().
>>> 
>>> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>
>>> ---
>> 
>> Looks good to me,
>> Reviewed-by: Harry Yoo <harry.yoo@...cle.com>
> 
> Thanks.
> 
>> 
>> with a nit below.
>> 
>>>  mm/slub.c | 79 ++++++++++++++++++++++++++++++++++++++++++++-------------------
>>>  1 file changed, 56 insertions(+), 23 deletions(-)
>>> 
>>> diff --git a/mm/slub.c b/mm/slub.c
>>> index 706cb6398f05..b385247c219f 100644
>>> --- a/mm/slub.c
>>> +++ b/mm/slub.c
>>> @@ -6703,7 +6735,7 @@ void slab_free(struct kmem_cache *s, struct slab *slab, void *object,
>>>  
>>>  	if (likely(!IS_ENABLED(CONFIG_NUMA) || slab_nid(slab) == numa_mem_id())
>>>  	    && likely(!slab_test_pfmemalloc(slab))) {
>>> -		if (likely(free_to_pcs(s, object)))
>>> +		if (likely(free_to_pcs(s, object, true)))
>>>  			return;
>>>  	}
>>>  
>>> @@ -6964,7 +6996,8 @@ void kfree_nolock(const void *object)
>>>  	 * since kasan quarantine takes locks and not supported from NMI.
>>>  	 */
>>>  	kasan_slab_free(s, x, false, false, /* skip quarantine */true);
>>> -	do_slab_free(s, slab, x, x, 0, _RET_IP_);
>>> +	if (!free_to_pcs(s, x, false))
>>> +		do_slab_free(s, slab, x, x, 0, _RET_IP_);
>>>  }
>> 
>> nit: Maybe it's not that common but should we bypass sheaves if
>> it's from remote NUMA node just like slab_free()?
> 
> Right, will do.

However that means sheaves will help less with the defer_free() avoidance
here. It becomes more obvious after "slab: remove the do_slab_free()
fastpath". All remote object frees will be deferred. Guess we can revisit
later if we see there are too many and have no better solution...

>>>  EXPORT_SYMBOL_GPL(kfree_nolock);
>>>  
>>> @@ -7516,7 +7549,7 @@ int kmem_cache_alloc_bulk_noprof(struct kmem_cache *s, gfp_t flags, size_t size,
>>>  		size--;
>>>  	}
>>>  
>>> -	i = alloc_from_pcs_bulk(s, size, p);
>>> +	i = alloc_from_pcs_bulk(s, flags, size, p);
>>>  
>>>  	if (i < size) { >  		/*
>>> 
>> 
>