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

Michal Hocko
2017-12-05T15:43:10Z

On Tue 05-12-17 18:34:59, Kirill Tkhai wrote:
> 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.

OK, so it is bound to _a_ process context. The oom killer will not know
about which process has consumed those objects but the effect will be at
least reduced to a memcg.

> 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.

Could you share your testing scenario and the way how the system behaved
during a heavy aio?

I am not saying the patch is wrong, I am just trying to undestand all
the consequences.
--
Michal Hocko
SUSE Labs