Date:   Thu, 23 Feb 2023 13:28:12 -0400
From:   Jason Gunthorpe <jgg@nvidia.com>
To:     "T.J. Mercier" <tjmercier@google.com>
Cc:     Alistair Popple <apopple@nvidia.com>, Tejun Heo <tj@kernel.org>,
        Michal Hocko <mhocko@suse.com>,
        Yosry Ahmed <yosryahmed@google.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>
Subject: Re: [PATCH 14/19] mm: Introduce a cgroup for pinned memory
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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.
 
> FWIW this is also the behavior I was trying to use to attribute
> dma-buffers to their original allocators. Whoever touches it first
> gets charged as long as the memory is alive somewhere.
>
> Can't we do the same thing for pins?

If pins are tracked independently from memcg then definately not,
a process in cgroup A should never be able to make a charge on cgroup
B as a matter of security.

If pins are part of the memcg then we can't always turn the pin
request in to a NOP - the current cgroup always has to be charged for
the memory. Otherwise what is the point from a security perspective?

Jason