Re: [BUG] mm/cgroupv2: memory.min may lead to an OOM error

[Lance Yang <ioworker0@gmail.com>]

Hi Michal,

Thanks a lot for clarifying!

On Fri, Aug 2, 2024 at 6:58 AM Michal Koutný <mkoutny@suse.com> wrote:
>
> Hello.
>
> On Thu, Aug 01, 2024 at 07:40:10PM GMT, Lance Yang <ioworker0@gmail.com> wrote:
> > However, if the child cgroup doesn't exist and we add a process to the 'test'
> > cgroup, then attempt to create a large file(2GB) using dd, we won't encounter
> > an OOM error; everything works as expected.
>
> That's due to the way how effective protections are calculated, see [1].
> If reclaim target is cgroup T, then it won't enjoy protection configured
> on itself, whereas the child of T is subject of ancestral reclaim hence
> the protection applies.

Makes sense to me.

>
> That would mean that in your 1st demo, it is test/memory.max that
> triggers reclaim and then failure to reclaim from test/test-child causes
> OOM in test.
> That's interesting since the (same) limit of test-child/memory.max
> should be evaluated first. I guess it is in your example there are
> actually two parallel processes (1321 and 1324) so some charges may
> randomly propagate to the upper test/memory.max limit.
>
> As explained above, the 2nd demo has same reclaim target but due to no
> nesting, protection is moot.

Ah, that clears it up. I appreciate the detailed explanation - thanks!

> I believe you could reproduce with merely
>
>         test/memory.max
>         test-child/memory.min

Yep, I just tested it, and you're right ;)

>
> > Hmm... I'm a bit confused about that.
>
> I agree, the calculation of effective protection wrt reclaim target can
> be confusing.
>
> The effects you see are documented for memory.min:
>
> > Putting more memory than generally available under this
> > protection is discouraged and may lead to constant OOMs.

Thanks a lot again for your time!
Lance

>
> HTH,
> Michal
>
> [1] https://lore.kernel.org/all/20200729140537.13345-2-mkoutny@suse.com/