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From:   Yosry Ahmed <yosryahmed@google.com>
Date:   Mon, 6 Feb 2023 14:39:17 -0800
Message-ID: <CAJD7tka6SC1ho-dffV0bK_acoZd-5DQzBOy0xg3TkOFG1zAPMg@mail.gmail.com>
Subject: Re: [PATCH 14/19] mm: Introduce a cgroup for pinned memory
To:     Tejun Heo <tj@kernel.org>
Cc:     Alistair Popple <apopple@nvidia.com>, linux-mm@kvack.org,
        cgroups@vger.kernel.org, linux-kernel@vger.kernel.org,
        jgg@nvidia.com, jhubbard@nvidia.com, tjmercier@google.com,
        hannes@cmpxchg.org, surenb@google.com, mkoutny@suse.com,
        daniel@ffwll.ch, "Daniel P . Berrange" <berrange@redhat.com>,
        Alex Williamson <alex.williamson@redhat.com>,
        Zefan Li <lizefan.x@bytedance.com>,
        Andrew Morton <akpm@linux-foundation.org>
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On Mon, Feb 6, 2023 at 2:36 PM Tejun Heo <tj@kernel.org> wrote:
>
> On Mon, Feb 06, 2023 at 02:32:10PM -0800, Yosry Ahmed wrote:
> > I guess it boils down to which we want:
> > (a) Limit the amount of memory processes in a cgroup can be pinned/locked.
> > (b) Limit the amount of memory charged to a cgroup that can be pinned/locked.
> >
> > The proposal is doing (a), I suppose if this was part of memcg it
> > would be (b), right?
> >
> > I am not saying it should be one or the other, I am just making sure
> > my understanding is clear.
>
> I don't quite understand what the distinction would mean in practice. It's
> just odd to put locked memory in a separate controller from interface POV.

Assume we have 2 cgroups, A and B. A process in cgroup A creates a
tmpfs file and writes to it, so the memory is now charged to cgroup A.
Now imagine a process in cgroup B tries to lock this memory.
- With (a) the amount of locked memory will count toward's cgroup A's
limit, because cgroup A is charged for the memory.
- With (b) the amount of locked memory will count toward's cgroup B's
limit, because a process in cgroup B is locking the memory.

I agree that it is confusing from an interface POV.

>
> Thanks.
>
> --
> tejun