Re: [PATCH v2 1/2] hugetlb: memcg: account hugetlb-backed memory in memory controller

[Yosry Ahmed]

On Fri, Sep 29, 2023 at 8:08 AM Johannes Weiner <hannes@xxxxxxxxxxx> wrote:
>
> On Thu, Sep 28, 2023 at 06:18:19PM -0700, Yosry Ahmed wrote:
> > My concern is the scenario where the memory controller is mounted in
> > cgroup v1, and cgroup v2 is mounted with memory_hugetlb_accounting.
> >
> > In this case it seems like the current code will only check whether
> > memory_hugetlb_accounting was set on cgroup v2 or not, disregarding
> > the fact that cgroup v1 did not enable hugetlb accounting.
> >
> > I obviously prefer that any features are also added to cgroup v1,
> > because we still didn't make it to cgroup v2, especially when the
> > infrastructure is shared. On the other hand, I am pretty sure the
> > maintainers will not like what I am saying :)
>
> I have a weak preference.
>
> It's definitely a little weird that the v1 controller's behavior
> changes based on the v2 mount flag. And that if you want it as an
> otherwise exclusive v1 user, you'd have to mount a dummy v2.
>
> But I also don't see a scenario where it would hurt, or where there
> would be an unresolvable conflict between v1 and v2 in expressing
> desired behavior, since the memory controller is exclusive to one.
>
> While we could eliminate this quirk with a simple
> !cgroup_subsys_on_dfl(memory_cgrp_subsys) inside the charge function,
> it would seem almost punitive to add extra code just to take something
> away that isn't really a problem and could be useful to some people.
>
> If Tejun doesn't object, I'd say let's just keep implied v1 behavior.

I agree that adding extra code to take a feature away from v1 is
probably too much, but I also think relying on a v2 mount option is
weird. Would it be too much to just have a v1-specific flag as well
and use cgroup_subsys_on_dfl(memory_cgrp_subsys) to decide which flag
to read?