linux-kernel - Re: [PATCH] slab: annotate on-slab caches nodelist locks

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Date:	Thu, 01 Nov 2012 17:10:28 +0800
From:	Michael Wang <wangyun@...ux.vnet.ibm.com>
To:	Glauber Costa <glommer@...allels.com>
CC:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Christoph Lameter <cl@...ux.com>,
	Pekka Enberg <penberg@...helsinki.fi>,
	David Rientjes <rientjes@...gle.com>,
	JoonSoo Kim <js1304@...il.com>
Subject: Re: [PATCH] slab: annotate on-slab caches nodelist locks

On 11/02/2012 12:48 AM, Glauber Costa wrote:
> On 11/01/2012 11:11 AM, Michael Wang wrote:
>> On 10/29/2012 06:49 PM, Glauber Costa wrote:
>>> We currently provide lockdep annotation for kmalloc caches, and also
>>> caches that have SLAB_DEBUG_OBJECTS enabled. The reason for this is that
>>> we can quite frequently nest in the l3->list_lock lock, which is not
>>> something trivial to avoid.
>>>
>>> My proposal with this patch, is to extend this to caches whose slab
>>> management object lives within the slab as well ("on_slab"). The need
>>> for this arose in the context of testing kmemcg-slab patches. With such
>>> patchset, we can have per-memcg kmalloc caches. So the same path that
>>> led to nesting between kmalloc caches will could then lead to in-memcg
>>> nesting. Because they are not annotated, lockdep will trigger.
>>
>> Hi, Glauber
>>
>> I'm trying to understand what's the issue we are trying to solve, but
>> looks like I need some help...
>>
> Understandably =)
> 
> This will not trigger in an upstream kernel, so in this sense, it is not
> an existing bug. It happens when the kmemcg-slab series is applied
> (https://lkml.org/lkml/2012/10/16/186) and (http://lwn.net/Articles/519877/)
> 
> Because this is a big series, I am for a while adopting the policy of
> sending out patches that are in principle independent of the series, to
> be reviewed on their own. But in some cases like this, some context may
> end up missing.
> 
> Now, of course I won't tell you to go read it all, so here is a summary:
> * We operate in a containerized environment, with each container inside
> a cgroup
> * in this context, it is necessary to account and limit the amount of
> kernel memory that can be tracked back to processes. This is akin of
> OpenVZ's beancounters (http://wiki.openvz.org/Proc/user_beancounters)
> * To do that, we create a version of each slab that a cgroup uses.
> Processes in that cgroup will allocate from that slab.
> 
> This means that we will have cgroup-specific versions of slabs like
> kmalloc-XX, dentry, inode, etc.
> 
>> So allow me to ask few questions:
>>
>> 1. what's scene will cause the fake dead lock?
> 
> This lockdep annotation exists because when freeing from kmalloc caches,
> it is possible to nest in the l3 list_lock. The particular one I hit was
> when we reach cache_flusharray with the l3 list_lock held, which seems
> to happen quite often.
> 
>> 2. what's the conflict caches?
> kmalloc-XX and kmalloc-memcg-y-XX
> 
>> 3. how does their lock operation nested?
>>
> 
> In the same way kmalloc-XX would nest with itself.

So this is a patch to fix the possible BUG if other patch applied?
I'm not sure but sounds like not the right process...add this one to
that patch set may be better :)

Regards,
Michael Wang

> 

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