linux-kernel - Re: [PATCH 14/19] mm: Introduce a cgroup for pinned memory

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Message-ID: <Y+GMbWWP/YhtJQqe@slm.duckdns.org>
Date:   Mon, 6 Feb 2023 13:25:33 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Yosry Ahmed <yosryahmed@...gle.com>
Cc:     Alistair Popple <apopple@...dia.com>, linux-mm@...ck.org,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
        jgg@...dia.com, jhubbard@...dia.com, tjmercier@...gle.com,
        hannes@...xchg.org, surenb@...gle.com, mkoutny@...e.com,
        daniel@...ll.ch, "Daniel P . Berrange" <berrange@...hat.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Zefan Li <lizefan.x@...edance.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 14/19] mm: Introduce a cgroup for pinned memory

On Mon, Feb 06, 2023 at 02:39:17PM -0800, Yosry Ahmed wrote:
> On Mon, Feb 6, 2023 at 2:36 PM Tejun Heo <tj@...nel.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.

Oh yeah, that's confusing. I'd go with (a) for consistency with the rest of
memcg - locked memory should fit inside e.g. memory.max. The problem with
shared memory accounting exists for non-locked memory as well and prolly
best to handle the same way rather than handling differently.

Thanks.

-- 
tejun