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

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Date:   Wed, 21 Nov 2018 03:02:41 +0000
From:   Wei Yang <richard.weiyang@...il.com>
To:     Wengang Wang <wen.gang.wang@...cle.com>
Cc:     Wei Yang <richard.weiyang@...il.com>, 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

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

-- 
Wei Yang
Help you, Help me