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From: Jason Gunthorpe <jgg@ziepe.ca>
To: Peter Xu <peterx@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	Leon Romanovsky <leonro@nvidia.com>,
	Linux-MM <linux-mm@kvack.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	"Maya B . Gokhale" <gokhale2@llnl.gov>,
	Yang Shi <yang.shi@linux.alibaba.com>,
	Marty Mcfadden <mcfadden8@llnl.gov>,
	Kirill Shutemov <kirill@shutemov.name>,
	Oleg Nesterov <oleg@redhat.com>, Jann Horn <jannh@google.com>,
	Jan Kara <jack@suse.cz>, Kirill Tkhai <ktkhai@virtuozzo.com>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Christoph Hellwig <hch@lst.de>,
	Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH 1/4] mm: Trial do_wp_page() simplification
Date: Tue, 15 Sep 2020 15:29:33 -0300
Message-ID: <20200915182933.GM1221970@ziepe.ca> (raw)
In-Reply-To: <20200915160553.GJ1221970@ziepe.ca>

On Tue, Sep 15, 2020 at 01:05:53PM -0300, Jason Gunthorpe wrote:
> On Tue, Sep 15, 2020 at 10:50:40AM -0400, Peter Xu wrote:
> > On Mon, Sep 14, 2020 at 08:28:51PM -0300, Jason Gunthorpe wrote:
> > > Yes, this stuff does pin_user_pages_fast() and MADV_DONTFORK
> > > together. It sets FOLL_FORCE and FOLL_WRITE to get an exclusive copy
> > > of the page and MADV_DONTFORK was needed to ensure that a future fork
> > > doesn't establish a COW that would break the DMA by moving the
> > > physical page over to the fork. DMA should stay with the process that
> > > called pin_user_pages_fast() (Is MADV_DONTFORK still needed with
> > > recent years work to GUP/etc? It is a pretty terrible ancient thing)
> > 
> > ... Now I'm more confused on what has happened.
> 
> I'm going to try to confirm that the MADV_DONTFORK is actually being
> done by userspace properly, more later.

It turns out the test is broken and does not call MADV_DONTFORK when
doing forks - it is an opt-in it didn't do.

It looks to me like this patch makes it much more likely that the COW
break after page pinning will end up moving the pinned physical page
to the fork while before it was not very common. Does that make sense?

Given that the tests are wrong it seems like broken userspace,
however, it also worked reliably for a fairly long time.

I'm waiting for confirmation that adding the missing MADV_DONTFORKS
restores all tests to success even with this patch.

Regards,
Jason

  reply index

Thread overview: 93+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-08-21 23:49 [PATCH 0/4] mm: Simplfy cow handling Peter Xu
2020-08-21 23:49 ` [PATCH 1/4] mm: Trial do_wp_page() simplification Peter Xu
2020-08-24  8:36   ` Kirill Tkhai
2020-08-24 14:30     ` Jan Kara
2020-08-24 15:37       ` Kirill Tkhai
2020-08-24 18:22         ` Linus Torvalds
2020-09-01  7:01           ` Hugh Dickins
2020-09-14 14:38   ` Jason Gunthorpe
2020-09-14 17:32     ` Linus Torvalds
2020-09-14 18:34       ` Peter Xu
2020-09-14 21:15         ` Peter Xu
2020-09-14 22:55           ` Jason Gunthorpe
2020-09-14 22:59             ` Linus Torvalds
2020-09-14 23:28               ` Jason Gunthorpe
2020-09-15  0:19                 ` Linus Torvalds
2020-09-15 14:50                 ` Peter Xu
2020-09-15 15:17                   ` Peter Xu
2020-09-15 16:05                   ` Jason Gunthorpe
2020-09-15 18:29                     ` Jason Gunthorpe [this message]
2020-09-15 19:13                       ` Peter Xu
2020-09-15 19:38                         ` Jason Gunthorpe
2020-09-15 21:33                           ` Peter Xu
2020-09-15 23:22                             ` Jason Gunthorpe
2020-09-16  1:50                               ` John Hubbard
2020-09-16 17:48                                 ` Jason Gunthorpe
2020-09-16 18:46                                   ` Peter Xu
2020-09-17 11:25                                     ` Jason Gunthorpe
2020-09-17 18:11                                       ` Linus Torvalds
2020-09-17 19:38                                         ` Jason Gunthorpe
2020-09-17 19:51                                           ` Linus Torvalds
2020-09-18 16:40                                             ` Peter Xu
2020-09-18 17:16                                               ` Linus Torvalds
2020-09-18 19:57                                                 ` Peter Xu
2020-09-18 17:32                                               ` Jason Gunthorpe
2020-09-18 20:40                                                 ` Peter Xu
2020-09-18 20:59                                                   ` Linus Torvalds
2020-09-19  0:28                                                     ` Jason Gunthorpe
2020-09-18 21:06                                                   ` John Hubbard
2020-09-19  0:01                                                     ` Jason Gunthorpe
2020-09-21  8:35                                                       ` Jan Kara
2020-09-21 12:03                                                         ` Jason Gunthorpe
2020-09-21 13:42                                               ` Michal Hocko
2020-09-21 14:18                                                 ` Peter Xu
2020-09-21 14:28                                                   ` Michal Hocko
2020-09-21 14:38                                                     ` Tejun Heo
2020-09-21 14:43                                                       ` Christian Brauner
2020-09-21 14:55                                                         ` Michal Hocko
2020-09-21 15:04                                                           ` Christian Brauner
2020-09-21 16:06                                                             ` Michal Hocko
2020-09-23  7:53                                                               ` Michal Hocko
2020-09-21 14:41                                                 ` Christian Brauner
2020-09-21 14:57                                                   ` Michal Hocko
2020-09-21 16:31                                                     ` Peter Xu
2020-09-17 18:14                                       ` Peter Xu
2020-09-17 18:26                                         ` Linus Torvalds
2020-09-17 19:03                                           ` Peter Xu
2020-09-17 19:42                                             ` Linus Torvalds
2020-09-17 19:55                                               ` John Hubbard
2020-09-17 20:06                                               ` Jason Gunthorpe
2020-09-17 20:19                                                 ` John Hubbard
2020-09-17 20:25                                                   ` Jason Gunthorpe
2020-09-17 20:35                                                 ` Linus Torvalds
2020-09-17 21:40                                                   ` Peter Xu
2020-09-17 22:09                                                     ` Jason Gunthorpe
2020-09-17 22:25                                                       ` Linus Torvalds
2020-09-17 22:48                                                       ` Ira Weiny
2020-09-18  9:36                                                         ` Jan Kara
2020-09-18  9:44                                                       ` Jan Kara
2020-09-18 16:19                                             ` Jason Gunthorpe
2020-09-15 10:23           ` Leon Romanovsky
2020-09-15 15:56           ` Jason Gunthorpe
2020-09-15 15:03   ` Oleg Nesterov
2020-09-15 16:18     ` Peter Xu
2020-08-21 23:49 ` [PATCH 2/4] mm/ksm: Remove reuse_ksm_page() Peter Xu
2020-08-21 23:49 ` [PATCH 3/4] mm/gup: Remove enfornced COW mechanism Peter Xu
2020-09-14 14:27   ` Oleg Nesterov
2020-09-14 17:59     ` Peter Xu
2020-09-14 19:03       ` Linus Torvalds
2020-08-21 23:49 ` [PATCH 4/4] mm: Add PGREUSE counter Peter Xu
2020-08-22 16:14   ` Linus Torvalds
2020-08-24  0:24     ` Peter Xu
2020-08-22 16:05 ` [PATCH 0/4] mm: Simplfy cow handling Linus Torvalds
2020-08-23 23:58   ` Peter Xu
2020-08-24  8:38 ` Kirill Tkhai
2020-08-27 14:15 ` Peter Xu
2021-02-02 14:40 [PATCH 1/4] mm: Trial do_wp_page() simplification Gal Pressman
2021-02-02 16:31 ` Peter Xu
2021-02-02 16:44   ` Jason Gunthorpe
2021-02-02 17:05     ` Peter Xu
2021-02-02 17:13       ` Jason Gunthorpe
2021-02-03 12:43         ` Gal Pressman
2021-02-03 14:00           ` Jason Gunthorpe
2021-02-03 14:47             ` Gal Pressman

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