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From: Jan Kara <jack@suse.cz>
To: john.hubbard@gmail.com
Cc: Matthew Wilcox <willy@infradead.org>,
	Michal Hocko <mhocko@kernel.org>,
	Christopher Lameter <cl@linux.com>,
	Jason Gunthorpe <jgg@ziepe.ca>,
	Dan Williams <dan.j.williams@intel.com>, Jan Kara <jack@suse.cz>,
	Al Viro <viro@zeniv.linux.org.uk>,
	linux-mm@kvack.org, LKML <linux-kernel@vger.kernel.org>,
	linux-rdma <linux-rdma@vger.kernel.org>,
	linux-fsdevel@vger.kernel.org, John Hubbard <jhubbard@nvidia.com>
Subject: Re: [PATCH 0/2] mm/fs: put_user_page() proposal
Date: Mon, 9 Jul 2018 18:27:58 +0200
Message-ID: <20180709162758.slewonsgbly4uhhg@quack2.suse.cz> (raw)
In-Reply-To: <20180709080554.21931-1-jhubbard@nvidia.com>

Hi,

On Mon 09-07-18 01:05:52, john.hubbard@gmail.com wrote:
> From: John Hubbard <jhubbard@nvidia.com>
> 
> With respect to tracking get_user_pages*() pages with page->dma_pinned*
> fields [1], I spent a few days retrofitting most of the get_user_pages*()
> call sites, by adding calls to a new put_user_page() function, in place
> of put_page(), where appropriate. This will work, but it's a large effort.
> 
> Design note: I didn't see anything that hinted at a way to fix this
> problem, without actually changing all of the get_user_pages*() call sites,
> so I think it's reasonable to start with that.

Agreed.

> Anyway, it's still incomplete, but because this is a large, tree-wide
> change (that will take some time and testing), I'd like to propose a plan,
> before spamming zillions of people with put_user_page() conversion patches.
> So I picked out the first two patches to show where this is going.
> 
> Proposed steps:
> 
> Step 1:
> 
> Start with the patches here, then continue with...dozens more.
> This will eventually convert all of the call sites to use put_user_page().
> This is easy in some places, but complex in others, such as:
> 
>     -- drivers/gpu/drm/amd
>     -- bio
>     -- fuse
>     -- cifs
>     -- anything from:
>            git grep  iov_iter_get_pages | cut -f1 -d ':' | sort | uniq
> 
> The easy ones can be grouped into a single patchset, perhaps, and the
> complex ones probably each need a patchset, in order to get the in-depth
> review they'll need.

Agreed.

> Furthermore, some of these areas I hope to attract some help on, once
> this starts going.
> 
> Step 2:
> 
> In parallel, tidy up the core patchset that was discussed in [1], (version
> 2 has already been reviewed, so I know what to do), and get it perfected
> and reviewed. Don't apply it until step 1 is all done, though.
>
> Step 3:
> 
> Activate refcounting of dma-pinned pages (essentially, patch #5, which is
> [1]), but don't use it yet. Place a few WARN_ON_ONCE calls to start
> mopping up any missed call sites.
> 
> Step 4:
> 
> After some soak time, actually connect it up (patch #6 of [1]) and start
> taking action based on the new page->dma_pinned* fields.
> 
> [1] https://www.spinics.net/lists/linux-mm/msg156409.html
> 
>   or, the same thread on LKML if it's working for you:
> 
>     https://lkml.org/lkml/2018/7/4/368

Yeah, but as Nick pointed out we have some more work to do in step 4 to
avoid deadlocks. Still there's a lot of work to do on which the direction
to progress is clear :).

								Honza

-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

      parent reply index

Thread overview: 16+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-07-09  8:05 john.hubbard
2018-07-09  8:05 ` [PATCH 1/2] mm: introduce put_user_page(), placeholder version john.hubbard
2018-07-09 10:08   ` kbuild test robot
2018-07-09 18:48     ` John Hubbard
2018-07-09 15:53   ` Jason Gunthorpe
2018-07-09 16:11     ` Jan Kara
2018-07-09  8:05 ` [PATCH 2/2] goldfish_pipe/mm: convert to the new put_user_page() call john.hubbard
2018-07-09  8:49 ` [PATCH 0/2] mm/fs: put_user_page() proposal Nicholas Piggin
2018-07-09 16:08   ` Jan Kara
2018-07-09 17:16     ` Matthew Wilcox
2018-07-09 19:47       ` Jan Kara
2018-07-09 19:56         ` Jason Gunthorpe
2018-07-10  7:51           ` Jan Kara
2018-07-09 20:00         ` Matthew Wilcox
2018-07-10  8:21           ` Jan Kara
2018-07-09 16:27 ` Jan Kara [this message]

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