Re: [RFC][2/3] Account and control virtual address space allocations (v2)

[Pavel Emelyanov <xemul@openvz.org>]

Balbir Singh wrote:
> Pavel Emelyanov wrote:
>> Balbir Singh wrote:
>>> Changelog v2
>>> ------------
>>> Change the accounting to what is already present in the kernel. Split
>>> the address space accounting into mem_cgroup_charge_as and
>>> mem_cgroup_uncharge_as. At the time of VM expansion, call
>>> mem_cgroup_cannot_expand_as to check if the new allocation will push
>>> us over the limit
>>>
>>> This patch implements accounting and control of virtual address space.
>>> Accounting is done when the virtual address space of any task/mm_struct
>>> belonging to the cgroup is incremented or decremented. This patch
>>> fails the expansion if the cgroup goes over its limit.
>>>
>>> TODOs
>>>
>>> 1. Only when CONFIG_MMU is enabled, is the virtual address space control
>>>    enabled. Should we do this for nommu cases as well? My suspicion is
>>>    that we don't have to.
>>>
>>> Signed-off-by: Balbir Singh <balbir@linux.vnet.ibm.com>
>>> ---
>>>
>>>  arch/ia64/kernel/perfmon.c  |    2 +
>>>  arch/x86/kernel/ptrace.c    |    7 +++
>>>  fs/exec.c                   |    2 +
>>>  include/linux/memcontrol.h  |   26 +++++++++++++
>>>  include/linux/res_counter.h |   19 ++++++++--
>>>  init/Kconfig                |    2 -
>>>  kernel/fork.c               |   17 +++++++--
>>>  mm/memcontrol.c             |   83 ++++++++++++++++++++++++++++++++++++++++++++
>>>  mm/mmap.c                   |   11 +++++
>>>  mm/mremap.c                 |    2 +
>>>  10 files changed, 163 insertions(+), 8 deletions(-)
>>>
>>> diff -puN mm/memcontrol.c~memory-controller-virtual-address-space-accounting-and-control mm/memcontrol.c
>>> --- linux-2.6.25-rc5/mm/memcontrol.c~memory-controller-virtual-address-space-accounting-and-control	2008-03-26 16:27:59.000000000 +0530
>>> +++ linux-2.6.25-rc5-balbir/mm/memcontrol.c	2008-03-27 00:18:16.000000000 +0530
>>> @@ -526,6 +526,76 @@ unsigned long mem_cgroup_isolate_pages(u
>>>  	return nr_taken;
>>>  }
>>>  
>>> +#ifdef CONFIG_CGROUP_MEM_RES_CTLR_AS
>>> +/*
>>> + * Charge the address space usage for cgroup. This routine is most
>>> + * likely to be called from places that expand the total_vm of a mm_struct.
>>> + */
>>> +void mem_cgroup_charge_as(struct mm_struct *mm, long nr_pages)
>>> +{
>>> +	struct mem_cgroup *mem;
>>> +
>>> +	if (mem_cgroup_subsys.disabled)
>>> +		return;
>>> +
>>> +	rcu_read_lock();
>>> +	mem = rcu_dereference(mm->mem_cgroup);
>>> +	css_get(&mem->css);
>>> +	rcu_read_unlock();
>>> +
>>> +	res_counter_charge(&mem->as_res, (nr_pages * PAGE_SIZE));
>>> +	css_put(&mem->css);
>> Why don't you check whether the counter is charged? This is
>> bad for two reasons:
>> 1. you allow for some growth above the limit (e.g. in expand_stack)
> 
> I was doing that earlier and then decided to keep the virtual address space code
> in sync with the RLIMIT_AS checking code in the kernel. If you see the flow, it
> closely resembles what we do with mm->total_vm and may_expand_vm().
> expand_stack() in turn calls acct_stack_growth() which calls may_expand_vm()

But this is racy! Look - you do expand_stack on two CPUs and the limit is
almost reached - so that there's room for a single expansion. In this case 
may_expand_vm will return true for both, since it only checks the limit, 
while the subsequent charge will fail on one of them, since it actually 
tries to raise the usage...

>> 2. you will undercharge it in the future when uncharging the
>>    vme, whose charge was failed and thus unaccounted.
> 
> Hmmm...  This should ideally never happen, since we do a may_expand_vm() before
> expanding the VM and in our case the virtual address space usage. I've not seen
> it during my runs either. But it is something to keep in mind.
>