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

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Date:   Wed, 22 Feb 2023 16:05:50 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     Alistair Popple <apopple@...dia.com>
Cc:     Jason Gunthorpe <jgg@...dia.com>, Tejun Heo <tj@...nel.org>,
        Michal Hocko <mhocko@...e.com>,
        Yosry Ahmed <yosryahmed@...gle.com>, linux-mm@...ck.org,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
        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 Thu, Feb 23, 2023 at 09:59:35AM +1100, Alistair Popple wrote:
> The idea was every driver already needs to allocate a pages array to
> pass to pin_user_pages(), and by necessity drivers have to keep a
> reference to the contents of that in one form or another. So
> conceptually the equivalent of:
> 
> struct vm_account {
>        struct list_head possible_pinners;
>        struct mem_cgroup *memcg;
>        struct pages **pages;
>        [...]
> };
> 
> Unpinnig involves finding a new owner by traversing the list of
> page->memcg_data->possible_pinners and iterating over *pages[] to figure
> out if that vm_account actually has this page pinned or not and could
> own it.
> 
> Agree this is costly though. And I don't think all drivers keep the
> array around so "iterating over *pages[]" may need to be a callback.

Is pinning in this context referring to FOLL_LONGTERM pins or any
FOLL_PIN?  In the latter case block based direct I/O does not keep
the pages array around, and also is absolutely not willing to pay
for the overhead.

For FOLL_LONGTERM the schemes sounds vaguely reasonable to me.