Date:   Fri, 15 Jul 2022 18:51:54 +0800
From:   Rongwei Wang <rongwei.wang@...ux.alibaba.com>
To:     Vlastimil Babka <vbabka@...e.cz>,
        Hyeonggon Yoo <42.hyeyoo@...il.com>
Cc:     David Rientjes <rientjes@...gle.com>, songmuchun@...edance.com,
        akpm@...ux-foundation.org, 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 7/15/22 6:33 PM, Vlastimil Babka wrote:
> On 7/15/22 10:05, Rongwei Wang wrote:
>>
>>
>> On 6/17/22 5:40 PM, Vlastimil Babka wrote:
>>> On 6/8/22 14:23, 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.
>>>
>> Hi, Vlastimil, sorry for missing your message long time.
> 
> Hi, no problem.
> 
>>> It also further complicates the already tricky code. I wonder if we should
>>> make more benefit from the fact that for kmem_cache_debug() caches we don't
>>> leave any slabs on percpu or percpu partial lists, and also in
>>> free_debug_processing() we aready take both list_lock and slab_lock. If we
>>> just did the freeing immediately there under those locks, we would be
>>> protected against other freeing cpus by that list_lock and don't need the
>>> double cmpxchg tricks.
>> enen, I'm not sure get your "don't need the double cmpxchg tricks" means
>> completely. What you want to say is that replace cmpxchg_double_slab() here
>> with following code when kmem_cache_debug(s)?
>>
>> __slab_lock(slab);
>> if (slab->freelist == freelist_old && slab->counters == counters_old){
>>      slab->freelist = freelist_new;
>>      slab->counters = counters_new;
>>      __slab_unlock(slab);
>>      local_irq_restore(flags);
>>      return true;
>> }
>> __slab_unlock(slab);
> 
> Pretty much, but it's more complicated.
Yes, actually, I think reuse cmpxchg_double_slab() here is more concise 
in code. I'm already finish this part of code, but hesitating whether to 
replace cmpxchg_double_slab().
> 
>> If I make mistakes for your words, please let me know.
>> Thanks!
>>>
>>> What about against allocating cpus? More tricky as those will currently end
>>> up privatizing the freelist via get_partial(), only to deactivate it again,
>>> so our list_lock+slab_lock in freeing path would not protect in the
>>> meanwhile. But the allocation is currently very inefficient for debug
>>> caches, as in get_partial() it will take the list_lock to take the slab from
>>> partial list and then in most cases again in deactivate_slab() to return it.
>> It seems that I need speed some time to eat these words. Anyway, thanks.
>>>
>>> If instead the allocation path for kmem_cache_debug() cache would take a
>>> single object from the partial list (not whole freelist) under list_lock, it
>>> would be ultimately more efficient, and protect against freeing using
>>> list_lock. Sounds like an idea worth trying to me?
>>
>> Hyeonggon had a similar advice that split freeing and allocating slab from
>> debugging, likes below:
>>
>>
>> __slab_alloc() {
>>      if (kmem_cache_debug(s))
>>          slab_alloc_debug()
>>      else
>>          ___slab_alloc()
>> }
>>
>> I guess that above code aims to solve your mentioned problem (idea)?
>>
>> slab_free() {
>>      if (kmem_cache_debug(s))
>>          slab_free_debug()
>>      else
>>          __do_slab_free()
>> }
>>
>> Currently, I only modify the code of freeing slab to fix the confusing
>> messages of "slabinfo -v". If you agree, I can try to realize above
>> mentioned slab_alloc_debug() code. Maybe it's also a challenge to me.
> 
> I already started working on this approach and hope to post a RFC soon.
OK, that's great.
> 
>> Thanks for your time.
>>
>>> And of course we would stop creating the 'validate' sysfs files for
>>> non-debug caches.

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