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From: Jerome Glisse <jglisse@redhat.com>
To: Jan Kara <jack@suse.cz>
Cc: John Hubbard <jhubbard@nvidia.com>,
	Matthew Wilcox <willy@infradead.org>,
	Dave Chinner <david@fromorbit.com>,
	Dan Williams <dan.j.williams@intel.com>,
	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@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: Tue, 15 Jan 2019 12:15:58 -0500
Message-ID: <20190115171557.GB3696@redhat.com> (raw)
Message-ID: <20190115171558.veJnApH99lRIWgtLwE3m7fmh8EKKPI76QXf6lvmIoCE@z> (raw)
In-Reply-To: <20190115080759.GC29524@quack2.suse.cz>

On Tue, Jan 15, 2019 at 09:07:59AM +0100, Jan Kara wrote:
> On Mon 14-01-19 12:21:25, Jerome Glisse wrote:
> > On Mon, Jan 14, 2019 at 03:54:47PM +0100, Jan Kara wrote:
> > > On Fri 11-01-19 19:06:08, John Hubbard wrote:
> > > > On 1/11/19 6:46 PM, Jerome Glisse wrote:
> > > > > On Fri, Jan 11, 2019 at 06:38:44PM -0800, John Hubbard wrote:
> > > > > [...]
> > > > > 
> > > > >>>> The other idea that you and Dan (and maybe others) pointed out was a debug
> > > > >>>> option, which we'll certainly need in order to safely convert all the call
> > > > >>>> sites. (Mirror the mappings at a different kernel offset, so that put_page()
> > > > >>>> and put_user_page() can verify that the right call was made.)  That will be
> > > > >>>> a separate patchset, as you recommended.
> > > > >>>>
> > > > >>>> I'll even go as far as recommending the page lock itself. I realize that this 
> > > > >>>> adds overhead to gup(), but we *must* hold off page_mkclean(), and I believe
> > > > >>>> that this (below) has similar overhead to the notes above--but is *much* easier
> > > > >>>> to verify correct. (If the page lock is unacceptable due to being so widely used,
> > > > >>>> then I'd recommend using another page bit to do the same thing.)
> > > > >>>
> > > > >>> Please page lock is pointless and it will not work for GUP fast. The above
> > > > >>> scheme do work and is fine. I spend the day again thinking about all memory
> > > > >>> ordering and i do not see any issues.
> > > > >>>
> > > > >>
> > > > >> Why is it that page lock cannot be used for gup fast, btw?
> > > > > 
> > > > > Well it can not happen within the preempt disable section. But after
> > > > > as a post pass before GUP_fast return and after reenabling preempt then
> > > > > it is fine like it would be for regular GUP. But locking page for GUP
> > > > > is also likely to slow down some workload (with direct-IO).
> > > > > 
> > > > 
> > > > Right, and so to crux of the matter: taking an uncontended page lock
> > > > involves pretty much the same set of operations that your approach does.
> > > > (If gup ends up contended with the page lock for other reasons than these
> > > > paths, that seems surprising.) I'd expect very similar performance.
> > > > 
> > > > But the page lock approach leads to really dramatically simpler code (and
> > > > code reviews, let's not forget). Any objection to my going that
> > > > direction, and keeping this idea as a Plan B? I think the next step will
> > > > be, once again, to gather some performance metrics, so maybe that will
> > > > help us decide.
> > > 
> > > FWIW I agree that using page lock for protecting page pinning (and thus
> > > avoid races with page_mkclean()) looks simpler to me as well and I'm not
> > > convinced there will be measurable difference to the more complex scheme
> > > with barriers Jerome suggests unless that page lock contended. Jerome is
> > > right that you cannot just do lock_page() in gup_fast() path. There you
> > > have to do trylock_page() and if that fails just bail out to the slow gup
> > > path.
> > > 
> > > Regarding places other than page_mkclean() that need to check pinned state:
> > > Definitely page migration will want to check whether the page is pinned or
> > > not so that it can deal differently with short-term page references vs
> > > longer-term pins.
> > > 
> > > Also there is one more idea I had how to record number of pins in the page:
> > > 
> > > #define PAGE_PIN_BIAS	1024
> > > 
> > > get_page_pin()
> > > 	atomic_add(&page->_refcount, PAGE_PIN_BIAS);
> > > 
> > > put_page_pin();
> > > 	atomic_add(&page->_refcount, -PAGE_PIN_BIAS);
> > > 
> > > page_pinned(page)
> > > 	(atomic_read(&page->_refcount) - page_mapcount(page)) > PAGE_PIN_BIAS
> > > 
> > > This is pretty trivial scheme. It still gives us 22-bits for page pins
> > > which should be plenty (but we should check for that and bail with error if
> > > it would overflow). Also there will be no false negatives and false
> > > positives only if there are more than 1024 non-page-table references to the
> > > page which I expect to be rare (we might want to also subtract
> > > hpage_nr_pages() for radix tree references to avoid excessive false
> > > positives for huge pages although at this point I don't think they would
> > > matter). Thoughts?
> > 
> > Racing PUP are as likely to cause issues:
> > 
> > CPU0                        | CPU1       | CPU2
> >                             |            |
> >                             | PUP()      |
> >     page_pinned(page)       |            |
> >       (page_count(page) -   |            |
> >        page_mapcount(page)) |            |
> >                             |            | GUP()
> > 
> > So here the refcount snap-shot does not include the second GUP and
> > we can have a false negative ie the page_pinned() will return false
> > because of the PUP happening just before on CPU1 despite the racing
> > GUP on CPU2 just after.
> > 
> > I believe only either lock or memory ordering with barrier can
> > guarantee that we do not miss GUP ie no false negative. Still the
> > bias idea might be usefull as with it we should not need a flag.
> 
> Right. We need similar synchronization (i.e., page lock or careful checks
> with memory barriers) if we want to get a reliable page pin information.
> 
> > So to make the above safe it would still need the page write back
> > double check that i described so that GUP back-off if it raced with
> > page_mkclean,clear_page_dirty_for_io and the fs write page call back
> > which call test_set_page_writeback() (yes it is very unlikely but
> > might still happen).
> 
> Agreed. So with page lock it would actually look like:
> 
> get_page_pin()
> 	lock_page(page);
> 	wait_for_stable_page();
> 	atomic_add(&page->_refcount, PAGE_PIN_BIAS);
> 	unlock_page(page);
> 
> And if we perform page_pinned() check under page lock, then if
> page_pinned() returned false, we are sure page is not and will not be
> pinned until we drop the page lock (and also until page writeback is
> completed if needed).
> 

So i still can't see anything wrong with that idea, i had similar
one in the past and diss-missed and i can't remember why :( But
thinking over and over i do not see any issue beside refcount wrap
around. Which is something that can happens today thought i don't
think it can be use in an evil way and we can catch it and be
loud about it.

So i think the following would be bullet proof:


get_page_pin()
    atomic_add(&page->_refcount, PAGE_PIN_BIAS);
    smp_wmb();
    if (PageWriteback(page)) {
        // back off
        atomic_add(&page->_refcount, -PAGE_PIN_BIAS);
        // re-enable preempt if in fast
        wait_on_page_writeback(page);
        goto retry;
    }

put_page_pin();
	atomic_add(&page->_refcount, -PAGE_PIN_BIAS);

page_pinned(page)
	(atomic_read(&page->_refcount) - page_mapcount(page)) > PAGE_PIN_BIAS

test_set_page_writeback()
    ...
    wb = TestSetPageWriteback(page)
    smp_mb();
    if (page_pinned(page)) {
        // report page as pinned to caller of test_set_page_writeback()
    }
    ...

This is text book memory barrier. Either get_page_pin() see racing
test_set_page_writeback() or test_set_page_writeback() see racing GUP


An optimization for GUP:
get_page_pin()
    pwp = PageWriteback(page);
    smp_rmb();
    waspinned = page_pinned(page);
    if (!waspinned && pwp) {
        // backoff
    }

    atomic_add(&page->_refcount, PAGE_PIN_BIAS);
    smp_wmb();
    if (PageWriteback(page)) {
        // back off
        atomic_add(&page->_refcount, -PAGE_PIN_BIAS);
        // re-enable preempt if in fast
        wait_on_page_writeback(page);
        goto retry;
    }

If page was not pin prior to this GUP than we can back off early.


Anyway i think this is better than mapcount. I started an analysis
of all places that were looking at mapcount a few of them would have
need an update if we were to increment mapcount with GUP.

I will go take a look at THP and hugetlbfs in respect to this just
to check for way to mitigate false positive.

Cheers,
Jérôme

  parent reply index

Thread overview: 213+ 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  2:59                                                       ` John Hubbard
2019-01-11 16:51                                                       ` Jerome Glisse
2019-01-11 16:51                                                         ` Jerome Glisse
2019-01-12  1:04                                                         ` John Hubbard
2019-01-12  1:04                                                           ` John Hubbard
2019-01-12  2:02                                                           ` Jerome Glisse
2019-01-12  2:02                                                             ` Jerome Glisse
2019-01-12  2:38                                                             ` John Hubbard
2019-01-12  2:38                                                               ` John Hubbard
2019-01-12  2:46                                                               ` Jerome Glisse
2019-01-12  2:46                                                                 ` Jerome Glisse
2019-01-12  3:06                                                                 ` John Hubbard
2019-01-12  3:06                                                                   ` John Hubbard
2019-01-12  3:25                                                                   ` Jerome Glisse
2019-01-12  3:25                                                                     ` Jerome Glisse
2019-01-12 20:46                                                                     ` John Hubbard
2019-01-12 20:46                                                                       ` John Hubbard
2019-01-14 14:54                                                                   ` Jan Kara
2019-01-14 14:54                                                                     ` Jan Kara
2019-01-14 17:21                                                                     ` Jerome Glisse
2019-01-14 17:21                                                                       ` Jerome Glisse
2019-01-14 19:09                                                                       ` John Hubbard
2019-01-14 19:09                                                                         ` John Hubbard
2019-01-15  8:34                                                                         ` Jan Kara
2019-01-15  8:34                                                                           ` Jan Kara
2019-01-15 21:39                                                                           ` John Hubbard
2019-01-15 21:39                                                                             ` John Hubbard
2019-01-15  8:07                                                                       ` Jan Kara
2019-01-15  8:07                                                                         ` Jan Kara
2019-01-15 17:15                                                                         ` Jerome Glisse [this message]
2019-01-15 17:15                                                                           ` Jerome Glisse
2019-01-15 21:56                                                                           ` John Hubbard
2019-01-15 21:56                                                                             ` John Hubbard
2019-01-15 22:12                                                                             ` Jerome Glisse
2019-01-15 22:12                                                                               ` Jerome Glisse
2019-01-16  0:44                                                                               ` John Hubbard
2019-01-16  0:44                                                                                 ` John Hubbard
2019-01-16  1:56                                                                                 ` Jerome Glisse
2019-01-16  1:56                                                                                   ` Jerome Glisse
2019-01-16  2:01                                                                                   ` Dan Williams
2019-01-16  2:01                                                                                     ` Dan Williams
2019-01-16  2:23                                                                                     ` Jerome Glisse
2019-01-16  2:23                                                                                       ` Jerome Glisse
2019-01-16  4:34                                                                                       ` Dave Chinner
2019-01-16  4:34                                                                                         ` Dave Chinner
2019-01-16 14:50                                                                                         ` Jerome Glisse
2019-01-16 14:50                                                                                           ` Jerome Glisse
2019-01-16 22:51                                                                                           ` Dave Chinner
2019-01-16 22:51                                                                                             ` Dave Chinner
2019-01-16 11:38                                                                         ` Jan Kara
2019-01-16 11:38                                                                           ` Jan Kara
2019-01-16 13:08                                                                           ` Jerome Glisse
2019-01-16 13:08                                                                             ` Jerome Glisse
2019-01-17  5:42                                                                             ` John Hubbard
2019-01-17  5:42                                                                               ` John Hubbard
2019-01-17 15:21                                                                               ` Jerome Glisse
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  9:30                                                                               ` Jan Kara
2019-01-17 15:17                                                                               ` Jerome Glisse
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  5:25                                                                           ` John Hubbard
2019-01-17  9:04                                                                           ` Jan Kara
2019-01-17  9:04                                                                             ` Jan Kara
2019-01-12  3:14                                                               ` Jerome Glisse
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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