Re: [RFC][2/3] Account and control virtual address space allocations

[Pavel Emelyanov <xemul@openvz.org>]

Balbir Singh wrote:
> Pavel Emelyanov wrote:
>> Balbir Singh wrote:
>>> Pavel Emelyanov wrote:
>>>> [snip]
>>>>
>>>>> +int mem_cgroup_update_as(struct mm_struct *mm, long nr_pages)
>>>>> +{
>>>>> +	int ret = 0;
>>>>> +	struct mem_cgroup *mem;
>>>>> +	if (mem_cgroup_subsys.disabled)
>>>>> +		return ret;
>>>>> +
>>>>> +	rcu_read_lock();
>>>>> +	mem = rcu_dereference(mm->mem_cgroup);
>>>>> +	css_get(&mem->css);
>>>>> +	rcu_read_unlock();
>>>>> +
>>>>> +	if (nr_pages > 0) {
>>>>> +		if (res_counter_charge(&mem->as_res, (nr_pages * PAGE_SIZE)))
>>>>> +			ret = 1;
>>>>> +	} else
>>>>> +		res_counter_uncharge(&mem->as_res, (-nr_pages * PAGE_SIZE));
>>>> No, please, no. Let's make two calls - mem_cgroup_charge_as and mem_cgroup_uncharge_as.
>>>>
>>>> [snip]
>>>>
>>> Yes, sure :)
>> Thanks :)
>>
>>>>> @@ -1117,6 +1117,9 @@ munmap_back:
>>>>>  		}
>>>>>  	}
>>>>>  
>>>>> +	if (mem_cgroup_update_as(mm, len >> PAGE_SHIFT))
>>>>> +		return -ENOMEM;
>>>>> +
>>>> Why not use existintg cap_vm_enough_memory and co?
>>>>
>>> I thought about it and almost used may_expand_vm(), but there is a slight catch
>>> there. With cap_vm_enough_memory() or security_vm_enough_memory(), they are
>>> called after total_vm has been calculated. In our case we need to keep the
>>> cgroups equivalent of total_vm up to date, and we do this in mem_cgorup_update_as.
>> So? What prevents us from using these hooks? :)
> 
> 1. We need to account total_vm usage of the task anyway. So why have two places,
>    one for accounting and second for control?

We still have two of them even placing hooks in each place manually.

Besides, putting the mem_cgroup_(un)charge_as() in these vm hooks will
1. save the number of places to patch
2. help keeping memcgroup consistent in case someone adds more places
   that expand tasks vm (arches, drivers) - in case we have our hooks
   celled from inside vm ones, we won't have to patch more.

> 2. These hooks are activated for conditionally invoked for vma's with VM_ACCOUNT
>    set.

This is a good point against. But, wrt my previous comment, can we handle 
this somehow?

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