Re: [PATCH RESEND] memcg: Free spare array to avoid memory leak

[Sha Zhengju <handai.szj@gmail.com>]

On 05/02/2012 05:03 AM, Andrew Morton wrote:
> On Thu, 19 Apr 2012 16:54:50 +0800
> Sha Zhengju<handai.szj@gmail.com>  wrote:
>
>> From: Sha Zhengju<handai.szj@taobao.com>
>>
>> When the last event is unregistered, there is no need to keep the spare
>> array anymore. So free it to avoid memory leak.
> How serious is this leak?  Is there any way in which it can be used to
> consume unbounded amounts of memory?

While registering events, the ->primary will apply for a larger array to 
store
the new threshold info and the ->spare holds the old primary space.
But once unregistering event, the ->primary and ->spare pointer will be 
swapped
after updating thresholds info. So if we have an eventfd with many(>1) 
thresholds
attached to it, mem_cgroup_usage_unregister_event() will finally leave 
->spare
holding a large array and have no chance to be freed.

I hope it is clear.

>> --- a/mm/memcontrol.c
>> +++ b/mm/memcontrol.c
>> @@ -4412,6 +4412,12 @@ static void mem_cgroup_usage_unregister_event(struct cgroup *cgrp,
>>   swap_buffers:
>>   	/* Swap primary and spare array */
>>   	thresholds->spare = thresholds->primary;
>> +	/* If all events are unregistered, free the spare array */
>> +	if (!new) {
>> +		kfree(thresholds->spare);
>> +		thresholds->spare = NULL;
>> +	}
>> +
>>   	rcu_assign_pointer(thresholds->primary, new);
>>
> The resulting code is really quite convoluted.  Try to read through it
> and follow the handling of ->primary and ->spare.  Head spins.
>
> What is the protocol here?  If ->primary is NULL then ->spare must also
> be NULL?
>

To be simple:  if new(->primary) is NULL, it means we are unregistering
the last threshold and there is no need to keep ->spare any more.
So give the ->spare array a chance to be freed.

Thanks,
Sha

> I'll apply the patch, although I don't (yet) have sufficient info to
> know which kernels it should be applied to.  Perhaps someone could
> revisit this code and see if it can be made more straightforward.
>
> .
>

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