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Subject: Re: [LSF/MM TOPIC] get_user_pages() pins in file mappings
To:     Jan Kara <jack@suse.cz>, <lsf-pc@lists.linux-foundation.org>
CC:     <linux-fsdevel@vger.kernel.org>, <linux-mm@kvack.org>,
        Dan Williams <dan.j.williams@intel.com>,
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References: <20190124090400.GE12184@quack2.suse.cz>
From:   John Hubbard <jhubbard@nvidia.com>
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On 1/24/19 1:04 AM, Jan Kara wrote:
> This is a joint proposal with Dan Williams, John Hubbard, and J=C3=A9r=C3=
=B4me
> Glisse.
>=20
> Last year we've talked with Dan about issues we have with filesystems and
> GUP [1]. The crux of the problem lies in the fact that there is no
> coordination (or even awareness) between filesystem working on a page (su=
ch
> as doing writeback) and GUP user modifying page contents and setting it
> dirty. This can (and we have user reports of this) lead to data corruptio=
n,
> kernel crashes, and other fun.
>=20
> Since last year we have worked together on solving these problems and we
> have explored couple dead ends as well as hopefully found solutions to so=
me
> of the partial problems. So I'd like to give some overview of where we
> stand and what remains to be solved and get thoughts from wider community
> about proposed solutions / problems to be solved.
>=20
> In particular we hope to have reasonably robust mechanism of identifying
> pages pinned by GUP (patches will be posted soon) - I'd like to run that =
by
> MM folks (unless discussion happens on mailing lists before LSF/MM). We
> also have ideas how filesystems should react to pinned page in their
> writepages methods - there will be some changes needed in some filesystem=
s
> to bounce the page if they need stable page contents. So I'd like to
> explain why we chose to do bouncing to fs people (i.e., why we cannot jus=
t
> wait, skip the page, do something else etc.) to save us from the same
> discussion with each fs separately and also hash out what the API for
> filesystems to do this should look like. Finally we plan to keep pinned
> page permanently dirty - again something I'd like to explain why we do th=
is
> and gather input from other people.
>=20
> This should be ideally shared MM + FS session.
>=20
> [1] https://lwn.net/Articles/753027/
>=20

Yes! I'd like to attend and discuss this, for sure.=20

Meanwhile, as usual, I'm a bit late on posting an updated RFC for the page
identification part, but that's coming very soon.


thanks,
--=20
John Hubbard
NVIDIA