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From:   Yosry Ahmed <yosryahmed@google.com>
Date:   Thu, 23 Feb 2023 10:14:04 -0800
Message-ID: <CAJD7tkZHZrxK_szH=5tdDZzhDTNXpBVD-e+79RzFmpp_ZYMcBA@mail.gmail.com>
Subject: Re: [PATCH 14/19] mm: Introduce a cgroup for pinned memory
To:     Jason Gunthorpe <jgg@nvidia.com>
Cc:     "T.J. Mercier" <tjmercier@google.com>,
        Alistair Popple <apopple@nvidia.com>,
        Tejun Heo <tj@kernel.org>, Michal Hocko <mhocko@suse.com>,
        linux-mm@kvack.org, cgroups@vger.kernel.org,
        linux-kernel@vger.kernel.org, jhubbard@nvidia.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 Thu, Feb 23, 2023 at 10:11 AM Jason Gunthorpe <jgg@nvidia.com> wrote:
>
> On Thu, Feb 23, 2023 at 10:03:50AM -0800, Yosry Ahmed wrote:
> > On Thu, Feb 23, 2023 at 9:28 AM Jason Gunthorpe <jgg@nvidia.com> wrote:
> > >
> > > On Thu, Feb 23, 2023 at 09:18:23AM -0800, T.J. Mercier wrote:
> > >
> > > > > Solving that problem means figuring out when every cgroup stops using
> > > > > the memory - pinning or not. That seems to be very costly.
> > > > >
> > > > This is the current behavior of accounting for memfds, and I suspect
> > > > any kind of shared memory.
> > > >
> > > > If cgroup A creates a memfd, maps and faults in pages, shares the
> > > > memfd with cgroup B and then A unmaps and closes the memfd, then
> > > > cgroup A is still charged for the pages it faulted in.
> > >
> > > As we discussed, as long as the memory is swappable then eventually
> > > memory pressure on cgroup A will evict the memfd pages and then cgroup
> > > B will swap it in and be charged for it.
> >
> > I am not familiar with memfd, but based on
> > mem_cgroup_swapin_charge_folio() it seems like if cgroup B swapped in
> > the pages they will remain charged to cgroup A, unless cgroup A is
> > removed/offlined. Am I missing something?
>
> Ah, I don't know, Tejun said:
>
> "but it can converge when page usage transfers across cgroups
> if needed."
>
> Which I assumed was swap related but I don't know how convergence
> works.

I believe that's the case for file-backed pages, but I do not believe
it's the case for swap-backed pages.

>
> Jason