Re: [PATCH] mm/memcontrol: stop resize loop if limit was changed again

[Michal Hocko]

On Wed 20-03-24 18:12:50, Michal Hocko wrote:
> On Wed 20-03-24 13:09:07, Waiman Long wrote:
> > 
> > On 3/20/24 06:03, Pavel Tikhomirov wrote:
> > > In memory_max_write() we first set memcg->memory.max and only then
> > > try to enforce it in loop. What if while we are in loop someone else
> > > have changed memcg->memory.max but we are still trying to enforce
> > > the old value? I believe this can lead to nasty consequence like getting
> > > an oom on perfectly fine cgroup within it's limits or excess reclaim.
> > 
> > Concurrent write to the same cgroup control file is not possible as the
> > underlying kernfs_open_file structure has a mutex that serialize access to
> > the file.
> 
> This is good to know and I was not aware of that. Thanks!

Btw. even if the interface itself is serialized then uncoordinated
userspace is still timing dependent so fundamentally racy.
-- 
Michal Hocko
SUSE Labs