linux-kernel - Re: [PATCH] mm: use this_cpu_cmpxchg_double in put_cpu

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Date:   Tue, 20 Nov 2018 19:18:13 -0800
From:   Wengang Wang <wen.gang.wang@...cle.com>
To:     Wei Yang <richard.weiyang@...il.com>
Cc:     cl@...ux.com, penberg@...nel.org, rientjes@...gle.com,
        iamjoonsoo.kim@....com, akpm@...ux-foundation.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: use this_cpu_cmpxchg_double in put_cpu_partial

Hi Wei,

I think you will receive my reply to Zhong, But I am copying my comments 
for that patch here (again):

Copy starts ==>

I am not sure if the patch you mentioned intended to fix the problem here.
With that patch the negative page->pobjects would become a large 
positive value,
it will win the compare with s->cpu_partial and go ahead to unfreeze the 
partial slabs.
Though it may be not a perfect fix for this issue, it really fixes (or 
workarounds) the issue here.
I'd like to skip my patch..

<=== Copy ends

thanks,

wengang


On 2018/11/20 19:02, Wei Yang wrote:
> On Tue, Nov 20, 2018 at 09:58:58AM -0800, Wengang Wang wrote:
>> Hi Wei,
>>
>>
>> On 2018/11/17 17:02, Wei Yang wrote:
>>> On Fri, Nov 16, 2018 at 05:33:35PM -0800, Wengang Wang wrote:
>>>> The this_cpu_cmpxchg makes the do-while loop pass as long as the
>>>> s->cpu_slab->partial as the same value. It doesn't care what happened to
>>>> that slab. Interrupt is not disabled, and new alloc/free can happen in the
>>> Well, I seems to understand your description.
>>>
>>> There are two slabs
>>>
>>>      * one which put_cpu_partial() trying to free an object
>>>      * one which is the first slab in cpu_partial list
>>>
>>> There is some tricky case, the first slab in cpu_partial list we
>>> reference to will change since interrupt is not disabled.
>> Yes, two slabs involved here just as you said above.
>> And yes, the case is really tricky, but it's there.
>>
>>>> interrupt handlers. Theoretically, after we have a reference to the it,
>>>                                                                    ^^^
>>> 							 one more word?
>> sorry, "the" should not be there.
>>
>>>> stored in _oldpage_, the first slab on the partial list on this CPU can be
>>>                                               ^^^
>>> One little suggestion here, mayby use cpu_partial would be more easy to
>>> understand. I confused this with the partial list in kmem_cache_node at
>>> the first time.  :-)
>> Right, making others understanding easily is very important. I just meant
>> cpu_partial.
>>
>>>> moved to kmem_cache_node and then moved to different kmem_cache_cpu and
>>>> then somehow can be added back as head to partial list of current
>>>> kmem_cache_cpu, though that is a very rare case. If that rare case really
>>> Actually, no matter what happens after the removal of the first slab in
>>> cpu_partial, it would leads to problem.
>> Maybe you are right, what I see is the problem on the page->pobjects.
>>
>>>> happened, the reading of oldpage->pobjects may get a 0xdead0000
>>>> unexpectedly, stored in _pobjects_, if the reading happens just after
>>>> another CPU removed the slab from kmem_cache_node, setting lru.prev to
>>>> LIST_POISON2 (0xdead000000000200). The wrong _pobjects_(negative) then
>>>> prevents slabs from being moved to kmem_cache_node and being finally freed.
>>>>
>>>> We see in a vmcore, there are 375210 slabs kept in the partial list of one
>>>> kmem_cache_cpu, but only 305 in-use objects in the same list for
>>>> kmalloc-2048 cache. We see negative values for page.pobjects, the last page
>>>> with negative _pobjects_ has the value of 0xdead0004, the next page looks
>>>> good (_pobjects is 1).
>>>>
>>>> For the fix, I wanted to call this_cpu_cmpxchg_double with
>>>> oldpage->pobjects, but failed due to size difference between
>>>> oldpage->pobjects and cpu_slab->partial. So I changed to call
>>>> this_cpu_cmpxchg_double with _tid_. I don't really want no alloc/free
>>>> happen in between, but just want to make sure the first slab did expereince
>>>> a remove and re-add. This patch is more to call for ideas.
>>> Maybe not an exact solution.
>>>
>>> I took a look into the code and change log.
>>>
>>> _tid_ is introduced by commit 8a5ec0ba42c4 ('Lockless (and preemptless)
>>> fastpaths for slub'), which is used to guard cpu_freelist. While we don't
>>> modify _tid_ when cpu_partial changes.
>>>
>>> May need another _tid_ for cpu_partial?
>> Right, _tid_ changes later than cpu_partial changes.
>>
>> As pointed out by Zhong Jiang, the pobjects issue is fixed by commit
> Where you discussed this issue? Any reference I could get a look?
>
>> e5d9998f3e09 (not sure if by side effect, see my replay there),
> I took a look at this commit e5d9998f3e09 ('slub: make ->cpu_partial
> unsigned int'), but not see some relationship between them.
>
> Would you mind show me a link or cc me in case you have further
> discussion?
>
> Thanks.
>
>> I'd skip this patch.?? If we found other problems regarding the change of
>> cpu_partial, let's fix them. What do you think?
>>
>> thanks,
>> wengang