[Devel] Re: [PATCH v5 06/14] memcg: kmem controller infrastructure

[Glauber Costa]

On 10/22/2012 04:51 PM, Michal Hocko wrote:
> [Sorry for the late reply]
> 
> On Mon 22-10-12 16:34:15, Glauber Costa wrote:
>> On 10/20/2012 12:34 AM, David Rientjes wrote:
>>> On Fri, 19 Oct 2012, Glauber Costa wrote:
>>>
>>>>>>> What about gfp & __GFP_FS?
>>>>>>>
>>>>>>
>>>>>> Do you intend to prevent or allow OOM under that flag? I personally
>>>>>> think that anything that accepts to be OOM-killed should have GFP_WAIT
>>>>>> set, so that ought to be enough.
>>>>>>
>>>>>
>>>>> The oom killer in the page allocator cannot trigger without __GFP_FS 
>>>>> because direct reclaim has little chance of being very successful and 
>>>>> thus we end up needlessly killing processes, and that tends to happen 
>>>>> quite a bit if we dont check for it.  Seems like this would also happen 
>>>>> with memcg if mem_cgroup_reclaim() has a large probability of failing?
>>>>>
>>>>
>>>> I can indeed see tests for GFP_FS in some key locations in mm/ before
>>>> calling the OOM Killer.
>>>>
>>>> Should I test for GFP_IO as well?
>>>
>>> It's not really necessary, if __GFP_IO isn't set then it wouldn't make 
>>> sense for __GFP_FS to be set.
>>>
>>>> If the idea is preventing OOM to
>>>> trigger for allocations that can write their pages back, how would you
>>>> feel about the following test:
>>>>
>>>> may_oom = (gfp & GFP_KERNEL) && !(gfp & __GFP_NORETRY) ?
>>>>
>>>
>>> I would simply copy the logic from the page allocator and only trigger oom 
>>> for __GFP_FS and !__GFP_NORETRY.
>>>
>>
>> That seems reasonable to me. Michal ?
> 
> Yes it makes sense to be consistent with the global case. While we are
> at it, do we need to consider PF_DUMPCORE resp. !__GFP_NOFAIL?
> 
at least from kmem, GFP_NOFAIL will not reach this codepath. We will
ditch it to the root in memcontrol.h