Re: [LSF/MM TOPIC] get_user_pages() pins in file mappings

From: John Hubbard <jhubbard@nvidia.com>
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>,
	Jerome Glisse <jglisse@redhat.com>
Subject: Re: [LSF/MM TOPIC] get_user_pages() pins in file mappings
Date: Fri, 25 Jan 2019 18:58:15 -0800	[thread overview]
Message-ID: <041368e5-7bca-4093-47da-13f1608b0692@nvidia.com> (raw)
In-Reply-To: <20190124090400.GE12184@quack2.suse.cz>

On 1/24/19 1:04 AM, Jan Kara wrote:
> This is a joint proposal with Dan Williams, John Hubbard, and Jérôme
> Glisse.
> 
> 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 (such
> 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 corruption,
> kernel crashes, and other fun.
> 
> 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 some
> 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.
> 
> 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 filesystems
> 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 just
> 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 this
> and gather input from other people.
> 
> This should be ideally shared MM + FS session.
> 
> [1] https://lwn.net/Articles/753027/
> 

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

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,
-- 
John Hubbard
NVIDIA