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From: Jason Gunthorpe <jgg@ziepe.ca>
To: Jan Kara <jack@suse.cz>
Cc: Matthew Wilcox <willy@infradead.org>,
	Nicholas Piggin <npiggin@gmail.com>,
	john.hubbard@gmail.com, Michal Hocko <mhocko@kernel.org>,
	Christopher Lameter <cl@linux.com>,
	Dan Williams <dan.j.williams@intel.com>,
	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 13:56:57 -0600
Message-ID: <20180709195657.GA29026@ziepe.ca> (raw)
In-Reply-To: <20180709194740.rymbt2fzohbdmpye@quack2.suse.cz>

On Mon, Jul 09, 2018 at 09:47:40PM +0200, Jan Kara wrote:
> On Mon 09-07-18 10:16:51, Matthew Wilcox wrote:
> > On Mon, Jul 09, 2018 at 06:08:06PM +0200, Jan Kara wrote:
> > > On Mon 09-07-18 18:49:37, Nicholas Piggin wrote:
> > > > The problem with blocking in clear_page_dirty_for_io is that the fs is
> > > > holding the page lock (or locks) and possibly others too. If you
> > > > expect to have a bunch of long term references hanging around on the
> > > > page, then there will be hangs and deadlocks everywhere. And if you do
> > > > not have such log term references, then page lock (or some similar lock
> > > > bit) for the duration of the DMA should be about enough?
> > > 
> > > There are two separate questions:
> > > 
> > > 1) How to identify pages pinned for DMA? We have no bit in struct page to
> > > use and we cannot reuse page lock as that immediately creates lock
> > > inversions e.g. in direct IO code (which could be fixed but then good luck
> > > with auditing all the other GUP users). Matthew had an idea and John
> > > implemented it based on removing page from LRU and using that space in
> > > struct page. So we at least have a way to identify pages that are pinned
> > > and can track their pin count.
> > > 
> > > 2) What to do when some page is pinned but we need to do e.g.
> > > clear_page_dirty_for_io(). After some more thinking I agree with you that
> > > just blocking waiting for page to unpin will create deadlocks like:
> > 
> > Why are we trying to writeback a page that is pinned?  It's presumed to
> > be continuously redirtied by its pinner.  We can't evict it.
> 
> So what should be a result of fsync(file), where some 'file' pages are
> pinned e.g. by running direct IO? If we just skip those pages, we'll lie to
> userspace that data was committed while it was not (and it's not only about
> data that has landed in those pages via DMA, you can have first 1k of a page
> modified by normal IO in parallel to DMA modifying second 1k chunk). If
> fsync(2) returns error, it would be really unexpected by userspace and most
> apps will just not handle that correctly. So what else can you do than
> block?

I think as a userspace I would expect the 'current content' to be
flushed without waiting..

If you block fsync() then anyone using a RDMA MR with it will just
dead lock. What happens if two processes open the same file and
one makes a MR and the other calls fsync()? Sounds bad.

Jason

  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 [this message]
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

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