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

[Glauber Costa]

On 11/01/2012 01:10 PM, Michael Wang wrote:
> 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 :)
> 

It is in the patchset. As I said, I've *also* (not exclusively) been
sending separately for a while patches that are potentially good on
their own (iow, have no code dependency with the rest of the series). In
some cases it help, in some, it doesn't

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