Sender: Tejun Heo <htejun@gmail.com>
Date:   Tue, 21 Feb 2023 09:49:12 -1000
From:   Tejun Heo <tj@kernel.org>
To:     Jason Gunthorpe <jgg@nvidia.com>
Cc:     Michal Hocko <mhocko@suse.com>,
        Yosry Ahmed <yosryahmed@google.com>,
        Alistair Popple <apopple@nvidia.com>, linux-mm@kvack.org,
        cgroups@vger.kernel.org, linux-kernel@vger.kernel.org,
        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>
Subject: Re: [PATCH 14/19] mm: Introduce a cgroup for pinned memory
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On Tue, Feb 21, 2023 at 09:45:15AM -1000, Tejun Heo wrote:
> > Multiple cgroup can pin the same page, so it is not as simple as just
> > transfering ownership, we need multi-ownership and to really fix the
> > memcg limitations with MAP_SHARED without an API impact.
> > 
> > You are right that pinning is really just a special case of
> > allocation, but there is a reason the memcg was left with weak support
> > for MAP_SHARED and changing that may be more than just hard but an
> > infeasible trade off..
> > 
> > At least I don't have a good idea how to even approach building a
> > reasonable datstructure that can track the number of
> > charges per-cgroup per page. :\
> 
> As I wrote above, I don't think the problem here is the case of pages being
> shared by multiple cgroups concurrently. We can leave that problem for
> another thread. However, if we want to support accounting and control of
> pinned memory, we really shouldn't introduce a fundmental discrepancy like
> the owner and pinner disagreeing with each other. At least conceptually, the
> solution is rather straight-forward - whoever pins a page should also claim
> the ownership of it.

Ah, sorry, I missed the part about multiple cgroups pinning the same page.
Yeah, I can't think of a good answer for that.

Thanks.

-- 
tejun