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Date:   Fri, 29 Sep 2023 13:42:21 -0400
From:   Johannes Weiner <hannes@cmpxchg.org>
To:     Yosry Ahmed <yosryahmed@google.com>
Cc:     Nhat Pham <nphamcs@gmail.com>, akpm@linux-foundation.org,
        riel@surriel.com, mhocko@kernel.org, roman.gushchin@linux.dev,
        shakeelb@google.com, muchun.song@linux.dev, tj@kernel.org,
        lizefan.x@bytedance.com, shuah@kernel.org, mike.kravetz@oracle.com,
        linux-mm@kvack.org, kernel-team@meta.com,
        linux-kernel@vger.kernel.org, cgroups@vger.kernel.org
Subject: Re: [PATCH v2 1/2] hugetlb: memcg: account hugetlb-backed memory in
 memory controller
Message-ID: <20230929174221.GA19137@cmpxchg.org>
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On Fri, Sep 29, 2023 at 08:11:54AM -0700, Yosry Ahmed wrote:
> On Fri, Sep 29, 2023 at 8:08 AM Johannes Weiner <hannes@cmpxchg.org> 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?

Yeah, let's not preemptively add explicit new features to cgroup1.

Since we agree the incidental support is weird, let's filter hugetlb
charging on cgroup_subsys_on_dfl(memory_cgrp_subsys) after all. If
somebody wants this for v1 - and it doesn't sound like Google is even
in that category according to Frank - they should send a separate
patch and we can go through all the reasons why switching to v2 is not
an option for them.