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

[Roman Gushchin <roman.gushchin@linux.dev>]

On Wed, Mar 20, 2024 at 06:55:05PM +0800, Pavel Tikhomirov wrote:
> 
> 
> On 20/03/2024 18:28, Michal Hocko wrote:
> > On Wed 20-03-24 18:03:30, 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.
> > 
> > I would argue that uncoordinated hard limit configuration can cause
> > problems on their own.
> 
> Sorry, didn't know that.
> 
> > Beside how is this any different from changing
> > the high limit while we are inside the reclaim loop?
> 
> I believe reclaim loop rereads limits on each iteration, e.g. in
> reclaim_high(), so it should always be enforcing the right limit.
> 
> > 
> > > We also have exactly the same thing in memory_high_write().
> > > 
> > > So let's stop enforcing old limits if we already have a new ones.
> > 
> > I do see any reasons why this would be harmful I just do not see why
> > this is a real thing or why the new behavior is any better for racing
> > updaters as those are not deterministic anyway. If you have any actual
> > usecase then more details would really help to justify this change.
> > 
> > The existing behavior makes some sense as it enforces the given limit
> > deterministically.
> 
> I don't have any actual problem, usecase or reproduce at hand, I only see a
> potential problem:

If the problem is only potential and also not very severe (it's not a crash
or memory corruption or something like this), I'd say let's keep things as
they are.

Thanks!