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

[Balbir Singh <balbir@linux.vnet.ibm.com>]

Pavel Emelyanov wrote:
> 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...
> 

Hmm... yes, possibly. Thanks for pointing this out. For a single mm_struct, the
check is done under mmap_sem(), so it's OK for processes. I suspect, I'll have
to go back to what I had earlier. I don't want to add a mutex to mem_cgroup,
that will hurt parallelism badly.

-- 
	Warm Regards,
	Balbir Singh
	Linux Technology Center
	IBM, ISTL

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