linux-kernel - Re: [PATCH 1/3] mm/slub: fix the race between validate_slab and slab

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Date:   Sat, 11 Jun 2022 12:04:22 +0800
From:   Rongwei Wang <rongwei.wang@...ux.alibaba.com>
To:     Christoph Lameter <cl@...two.de>
Cc:     David Rientjes <rientjes@...gle.com>, songmuchun@...edance.com,
        Hyeonggon Yoo <42.hyeyoo@...il.com>, akpm@...ux-foundation.org,
        vbabka@...e.cz, roman.gushchin@...ux.dev, iamjoonsoo.kim@....com,
        penberg@...nel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] mm/slub: fix the race between validate_slab and
 slab_free



On 6/8/22 8:23 PM, Christoph Lameter wrote:
> On Wed, 8 Jun 2022, Rongwei Wang wrote:
> 
>> If available, I think document the issue and warn this incorrect behavior is
>> OK. But it still prints a large amount of confusing messages, and disturbs us?
> 
> Correct it would be great if you could fix this in a way that does not
> impact performance.
> 
>>> are current operations on the slab being validated.
>> And I am trying to fix it in following way. In a short, these changes only
>> works under the slub debug mode, and not affects the normal mode (I'm not
>> sure). It looks not elegant enough. And if all approve of this way, I can
>> submit the next version.
> 
> 
>>
>> Anyway, thanks for your time:).
>> -wrw
>>
>> @@ -3304,7 +3300,7 @@ static void __slab_free(struct kmem_cache *s,
> struct
>> slab *slab,
>>
>>   {
>>          void *prior;
>> -       int was_frozen;
>> +       int was_frozen, to_take_off = 0;
>>          struct slab new;
> 
> to_take_off has the role of !n ? Why is that needed?
> 
>> -       do {
>> -               if (unlikely(n)) {
>> +               spin_lock_irqsave(&n->list_lock, flags);
>> +               ret = free_debug_processing(s, slab, head, tail, cnt, addr);
> 
> Ok so the idea is to take the lock only if kmem_cache_debug. That looks
> ok. But it still adds a number of new branches etc to the free loop.
> 
> Some performance tests would be useful.
Hi Christoph

Thanks for your time!
Do you have some advice in benchmarks that need me to test? And I find 
that hackbench and lkp was used frequently in mm/slub.c commits[1,2]. 
But I have no idea how to use these two benchmarks test to cover the 
above changes. Can you give some examples? Thanks very much!

Sorry for late reply!

[1]https://lore.kernel.org/lkml/20210301080404.GF12822@xsang-OptiPlex-9020/
[2]https://lore.kernel.org/linux-mm/20210128134512.GF3592@techsingularity.net/