Re: [PATCH] aio: Add memcg accounting of user used data

[Kirill Tkhai]

On 05.12.2017 18:15, Michal Hocko wrote:
> On Tue 05-12-17 13:00:54, Kirill Tkhai wrote:
>> Currently, number of available aio requests may be
>> limited only globally. There are two sysctl variables
>> aio_max_nr and aio_nr, which implement the limitation
>> and request accounting. They help to avoid
>> the situation, when all the memory is eaten in-flight
>> requests, which are written by slow block device,
>> and which can't be reclaimed by shrinker.
>>
>> This meets the problem in case of many containers
>> are used on the hardware node. Since aio_max_nr is
>> a global limit, any container may occupy the whole
>> available aio requests, and to deprive others the
>> possibility to use aio at all. The situation may
>> happen because of evil intentions of the container's
>> user or because of the program error, when the user
>> makes this occasionally
>>
>> The patch allows to fix the problem. It adds memcg
>> accounting of user used aio data (the biggest is
>> the bunch of aio_kiocb; ring buffer is the second
>> biggest), so a user of a certain memcg won't be able
>> to allocate more aio requests memory, then the cgroup
>> allows, and he will bumped into the limit.
> 
> So what happens when we hit the hard limit and oom kill somebody?
> Are those charged objects somehow bound to a process context?

There is exit_aio() called from __mmput(), which waits till
the charged objects complete and decrement reference counter.

If there was a problem with oom in memcg, there would be
the same problem on global oom, as it can be seen there is
no __GFP_NOFAIL flags anywhere in aio code.

But it seems everything is safe.

Kirill