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

[Glauber Costa <glommer@parallels.com>]

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.

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