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From: Dave Chinner <david@fromorbit.com>
To: Jerome Glisse <jglisse@redhat.com>
Cc: Dan Williams <dan.j.williams@intel.com>,
	John Hubbard <jhubbard@nvidia.com>, Jan Kara <jack@suse.cz>,
	Matthew Wilcox <willy@infradead.org>,
	John Hubbard <john.hubbard@gmail.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Linux MM <linux-mm@kvack.org>,
	tom@talpey.com, Al Viro <viro@zeniv.linux.org.uk>,
	benve@cisco.com, Christoph Hellwig <hch@infradead.org>,
	Christopher Lameter <cl@linux.com>,
	"Dalessandro, Dennis" <dennis.dalessandro@intel.com>,
	Doug Ledford <dledford@redhat.com>,
	Jason Gunthorpe <jgg@ziepe.ca>, Michal Hocko <mhocko@kernel.org>,
	Mike Marciniszyn <mike.marciniszyn@intel.com>,
	rcampbell@nvidia.com,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>
Subject: Re: [PATCH 1/2] mm: introduce put_user_page*(), placeholder versions
Date: Wed, 16 Jan 2019 15:34:55 +1100
Message-ID: <20190116043455.GP4205@dastard> (raw)
In-Reply-To: <20190116022312.GJ3696@redhat.com>

On Tue, Jan 15, 2019 at 09:23:12PM -0500, Jerome Glisse wrote:
> On Tue, Jan 15, 2019 at 06:01:09PM -0800, Dan Williams wrote:
> > On Tue, Jan 15, 2019 at 5:56 PM Jerome Glisse <jglisse@redhat.com> wrote:
> > > On Tue, Jan 15, 2019 at 04:44:41PM -0800, John Hubbard wrote:
> > [..]
> > > To make it clear.
> > >
> > > Lock code:
> > >     GUP()
> > >         ...
> > >         lock_page(page);
> > >         if (PageWriteback(page)) {
> > >             unlock_page(page);
> > >             wait_stable_page(page);
> > >             goto retry;
> > >         }
> > >         atomic_add(page->refcount, PAGE_PIN_BIAS);
> > >         unlock_page(page);
> > >
> > >     test_set_page_writeback()
> > >         bool pinned = false;
> > >         ...
> > >         pinned = page_is_pin(page); // could be after TestSetPageWriteback
> > >         TestSetPageWriteback(page);
> > >         ...
> > >         return pinned;
> > >
> > > Memory barrier:
> > >     GUP()
> > >         ...
> > >         atomic_add(page->refcount, PAGE_PIN_BIAS);
> > >         smp_mb();
> > >         if (PageWriteback(page)) {
> > >             atomic_add(page->refcount, -PAGE_PIN_BIAS);
> > >             wait_stable_page(page);
> > >             goto retry;
> > >         }
> > >
> > >     test_set_page_writeback()
> > >         bool pinned = false;
> > >         ...
> > >         TestSetPageWriteback(page);
> > >         smp_wmb();
> > >         pinned = page_is_pin(page);
> > >         ...
> > >         return pinned;
> > >
> > >
> > > One is not more complex than the other. One can contend, the other
> > > will _never_ contend.
> > 
> > The complexity is in the validation of lockless algorithms. It's
> > easier to reason about locks than barriers for the long term
> > maintainability of this code. I'm with Jan and John on wanting to
> > explore lock_page() before a barrier-based scheme.
> 
> How is the above hard to validate ?

Well, if you think it's so easy, then please write the test cases so
we can add them to fstests and make sure that we don't break it in
future.

If you can't write filesystem test cases that exercise these race
conditions reliably, then the answer to your question is "it is
extremely hard to validate" and the correct thing to do is to start
with the simple lock_page() based algorithm.

Premature optimisation in code this complex is something we really,
really need to avoid.

-Dave.
-- 
Dave Chinner
david@fromorbit.com

  reply index

Thread overview: 179+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-12-04  0:17 [PATCH 0/2] put_user_page*(): start converting the call sites john.hubbard
2018-12-04  0:17 ` [PATCH 1/2] mm: introduce put_user_page*(), placeholder versions john.hubbard
2018-12-04  7:53   ` Mike Rapoport
2018-12-05  1:40     ` John Hubbard
2018-12-04 20:28   ` Dan Williams
2018-12-04 21:56     ` John Hubbard
2018-12-04 23:03       ` Dan Williams
2018-12-05  0:36         ` Jerome Glisse
2018-12-05  0:40           ` Dan Williams
2018-12-05  0:59             ` John Hubbard
2018-12-05  0:58         ` John Hubbard
2018-12-05  1:00           ` Dan Williams
2018-12-05  1:15           ` Matthew Wilcox
2018-12-05  1:44             ` Jerome Glisse
2018-12-05  1:57               ` John Hubbard
2018-12-07  2:45                 ` John Hubbard
2018-12-07 19:16                   ` Jerome Glisse
2018-12-07 19:26                     ` Dan Williams
2018-12-07 19:40                       ` Jerome Glisse
2018-12-08  0:52                     ` John Hubbard
2018-12-08  2:24                       ` Jerome Glisse
2018-12-10 10:28                         ` Jan Kara
2018-12-12 15:03                           ` Jerome Glisse
2018-12-12 16:27                             ` Dan Williams
2018-12-12 17:02                               ` Jerome Glisse
2018-12-12 17:49                                 ` Dan Williams
2018-12-12 19:07                                   ` John Hubbard
2018-12-12 21:30                               ` Jerome Glisse
2018-12-12 21:40                                 ` Dan Williams
2018-12-12 21:53                                   ` Jerome Glisse
2018-12-12 22:11                                     ` Matthew Wilcox
2018-12-12 22:16                                       ` Jerome Glisse
2018-12-12 23:37                                     ` Jason Gunthorpe
2018-12-12 23:46                                       ` John Hubbard
2018-12-12 23:54                                       ` Dan Williams
2018-12-13  0:01                                       ` Jerome Glisse
2018-12-13  0:18                                         ` Dan Williams
2018-12-13  0:44                                           ` Jerome Glisse
2018-12-13  3:26                                             ` Jason Gunthorpe
2018-12-13  3:20                                         ` Jason Gunthorpe
2018-12-13 12:43                                           ` Jerome Glisse
2018-12-13 13:40                                             ` Tom Talpey
2018-12-13 14:18                                               ` Jerome Glisse
2018-12-13 14:51                                                 ` Tom Talpey
2018-12-13 15:18                                                   ` Jerome Glisse
2018-12-13 18:12                                                     ` Tom Talpey
2018-12-13 19:18                                                       ` Jerome Glisse
2018-12-14 10:41                                             ` Jan Kara
2018-12-14 15:25                                               ` Jerome Glisse
2018-12-12 21:56                                 ` John Hubbard
2018-12-12 22:04                                   ` Jerome Glisse
2018-12-12 22:11                                     ` John Hubbard
2018-12-12 22:14                                       ` Jerome Glisse
2018-12-12 22:17                                         ` John Hubbard
2018-12-12 21:46                             ` Dave Chinner
2018-12-12 21:59                               ` Jerome Glisse
2018-12-13  0:51                                 ` Dave Chinner
2018-12-13  2:02                                   ` Jerome Glisse
2018-12-13 15:56                                     ` Christopher Lameter
2018-12-13 16:02                                       ` Jerome Glisse
2018-12-14  6:00                                     ` Dave Chinner
2018-12-14 15:13                                       ` Jerome Glisse
2018-12-14  3:52                                   ` John Hubbard
2018-12-14  5:21                                     ` Dan Williams
2018-12-14  6:11                                       ` John Hubbard
2018-12-14 15:20                                         ` Jerome Glisse
2018-12-14 19:38                                         ` Dan Williams
2018-12-14 19:48                                           ` Matthew Wilcox
2018-12-14 19:53                                             ` Dave Hansen
2018-12-14 20:03                                               ` Matthew Wilcox
2018-12-14 20:17                                                 ` Dan Williams
2018-12-14 20:29                                                   ` Matthew Wilcox
2018-12-15  0:41                                                 ` John Hubbard
2018-12-17  8:56                                           ` Jan Kara
2018-12-17 18:28                                             ` Dan Williams
2018-12-14 15:43                               ` Jan Kara
2018-12-16 21:58                                 ` Dave Chinner
2018-12-17 18:11                                   ` Jerome Glisse
2018-12-17 18:34                                     ` Matthew Wilcox
2018-12-17 19:48                                       ` Jerome Glisse
2018-12-17 19:51                                         ` Matthew Wilcox
2018-12-17 19:54                                           ` Jerome Glisse
2018-12-17 19:59                                             ` Matthew Wilcox
2018-12-17 20:55                                               ` Jerome Glisse
2018-12-17 21:03                                                 ` Matthew Wilcox
2018-12-17 21:15                                                   ` Jerome Glisse
2018-12-18  1:09                                       ` Dave Chinner
2018-12-18  6:12                                       ` Darrick J. Wong
2018-12-18  9:30                                       ` Jan Kara
2018-12-18 23:29                                         ` John Hubbard
2018-12-19  2:07                                           ` Jerome Glisse
2018-12-19 11:08                                             ` Jan Kara
2018-12-20 10:54                                               ` John Hubbard
2018-12-20 16:50                                                 ` Jerome Glisse
2018-12-20 16:57                                                   ` Dan Williams
2018-12-20 16:49                                               ` Jerome Glisse
2019-01-03  1:55                                               ` Jerome Glisse
2019-01-03  3:27                                                 ` John Hubbard
2019-01-03 14:57                                                   ` Jerome Glisse
2019-01-03  9:26                                                 ` Jan Kara
2019-01-03 14:44                                                   ` Jerome Glisse
2019-01-11  2:59                                                     ` John Hubbard
2019-01-11 16:51                                                       ` Jerome Glisse
2019-01-12  1:04                                                         ` John Hubbard
2019-01-12  2:02                                                           ` Jerome Glisse
2019-01-12  2:38                                                             ` John Hubbard
2019-01-12  2:46                                                               ` Jerome Glisse
2019-01-12  3:06                                                                 ` John Hubbard
2019-01-12  3:25                                                                   ` Jerome Glisse
2019-01-12 20:46                                                                     ` John Hubbard
2019-01-14 14:54                                                                   ` Jan Kara
2019-01-14 17:21                                                                     ` Jerome Glisse
2019-01-14 19:09                                                                       ` John Hubbard
2019-01-15  8:34                                                                         ` Jan Kara
2019-01-15 21:39                                                                           ` John Hubbard
2019-01-15  8:07                                                                       ` Jan Kara
2019-01-15 17:15                                                                         ` Jerome Glisse
2019-01-15 21:56                                                                           ` John Hubbard
2019-01-15 22:12                                                                             ` Jerome Glisse
2019-01-16  0:44                                                                               ` John Hubbard
2019-01-16  1:56                                                                                 ` Jerome Glisse
2019-01-16  2:01                                                                                   ` Dan Williams
2019-01-16  2:23                                                                                     ` Jerome Glisse
2019-01-16  4:34                                                                                       ` Dave Chinner [this message]
2019-01-16 14:50                                                                                         ` Jerome Glisse
2019-01-16 22:51                                                                                           ` Dave Chinner
2019-01-16 11:38                                                                         ` Jan Kara
2019-01-16 13:08                                                                           ` Jerome Glisse
2019-01-17  5:42                                                                             ` John Hubbard
2019-01-17 15:21                                                                               ` Jerome Glisse
2019-01-18  0:16                                                                                 ` Dave Chinner
2019-01-18  1:59                                                                                   ` Jerome Glisse
2019-01-17  9:30                                                                             ` Jan Kara
2019-01-17 15:17                                                                               ` Jerome Glisse
2019-01-22 15:24                                                                                 ` Jan Kara
2019-01-22 16:46                                                                                   ` Jerome Glisse
2019-01-23 18:02                                                                                     ` Jan Kara
2019-01-23 19:04                                                                                       ` Jerome Glisse
2019-01-29  0:22                                                                                         ` John Hubbard
2019-01-29  1:23                                                                                           ` Jerome Glisse
2019-01-29  6:41                                                                                             ` John Hubbard
2019-01-29 10:12                                                                                               ` Jan Kara
2019-01-30  2:21                                                                                                 ` John Hubbard
2019-01-17  5:25                                                                         ` John Hubbard
2019-01-17  9:04                                                                           ` Jan Kara
2019-01-12  3:14                                                               ` Jerome Glisse
2018-12-18 10:33                                   ` Jan Kara
2018-12-18 23:42                                     ` Dave Chinner
2018-12-19  3:03                                       ` Jason Gunthorpe
2018-12-19  5:26                                         ` Dan Williams
2018-12-19 11:19                                           ` Jan Kara
2018-12-19 10:28                                         ` Dave Chinner
2018-12-19 11:35                                           ` Jan Kara
2018-12-19 16:56                                             ` Jason Gunthorpe
2018-12-19 22:33                                             ` Dave Chinner
2018-12-20  9:07                                               ` Jan Kara
2018-12-20 16:54                                               ` Jerome Glisse
2018-12-19 13:24                                       ` Jan Kara
2018-12-08  5:18                       ` Matthew Wilcox
2018-12-12 19:13                         ` John Hubbard
2018-12-08  7:16                       ` Dan Williams
2018-12-08 16:33                         ` Jerome Glisse
2018-12-08 16:48                           ` Christoph Hellwig
2018-12-08 17:47                             ` Jerome Glisse
2018-12-08 18:26                               ` Christoph Hellwig
2018-12-08 18:45                                 ` Jerome Glisse
2018-12-08 18:09                             ` Dan Williams
2018-12-08 18:12                               ` Christoph Hellwig
2018-12-11  6:18                               ` Dave Chinner
2018-12-05  5:52             ` Dan Williams
2018-12-05 11:16       ` Jan Kara
2018-12-04  0:17 ` [PATCH 2/2] infiniband/mm: convert put_page() to put_user_page*() john.hubbard
2018-12-04 17:10 ` [PATCH 0/2] put_user_page*(): start converting the call sites David Laight
2018-12-05  1:05   ` John Hubbard
2018-12-05 14:08     ` David Laight
2018-12-28  8:37       ` Pavel Machek
2019-02-08  7:56 [PATCH 0/2] mm: put_user_page() call site conversion first john.hubbard
2019-02-08  7:56 ` [PATCH 1/2] mm: introduce put_user_page*(), placeholder versions john.hubbard
2019-02-08 10:32   ` Mike Rapoport
2019-02-08 20:44     ` John Hubbard

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