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From: John Hubbard <jhubbard@nvidia.com>
To: Jann Horn <jannh@google.com>, Peter Xu <peterx@redhat.com>
Cc: Linux-MM <linux-mm@kvack.org>,
	kernel list <linux-kernel@vger.kernel.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Michal Hocko <mhocko@suse.com>,
	Kirill Shutemov <kirill@shutemov.name>,
	Oleg Nesterov <oleg@redhat.com>,
	Kirill Tkhai <ktkhai@virtuozzo.com>,
	Hugh Dickins <hughd@google.com>,
	Leon Romanovsky <leonro@nvidia.com>, Jan Kara <jack@suse.cz>,
	Christoph Hellwig <hch@lst.de>,
	Andrew Morton <akpm@linux-foundation.org>,
	Jason Gunthorpe <jgg@ziepe.ca>,
	Andrea Arcangeli <aarcange@redhat.com>
Subject: Re: [PATCH 4/5] mm: Do early cow for pinned pages during fork() for ptes
Date: Mon, 21 Sep 2020 15:18:27 -0700	[thread overview]
Message-ID: <07bc5f59-74ae-73e8-2616-f11712c27b58@nvidia.com> (raw)
In-Reply-To: <CAG48ez3frkqQNHbE5bEB6rwYdbyoAA3B9FQZo=HKkUzWCM4H0Q@mail.gmail.com>

On 9/21/20 2:55 PM, Jann Horn wrote:
> On Mon, Sep 21, 2020 at 11:20 PM Peter Xu <peterx@redhat.com> wrote:
...
> I dislike the whole pin_user_pages() concept because (as far as I
> understand) it fundamentally tries to fix a problem in the subset of
> cases that are more likely to occur in practice (long-term pins
> overlapping with things like writeback), and ignores the rarer cases
> ("short-term" GUP).
> 

Well, no, that's not really fair. pin_user_pages() provides a key
prerequisite to fixing *all* of the bugs in that area, not just a
subset. The 5 cases in Documentation/core-api/pin_user_pages.rst cover
this pretty well. Or if they don't, let me know and I'll have another
pass at it.

The case for a "pin count" that is (logically) separate from a
page->_refcount is real, and it fixes real problems. An elevated
refcount can be caused by a lot of things, but it can normally be waited
for and/or retried. The FOLL_PIN pages cannot.

Of course, a valid remaining criticism of the situation is, "why not
just *always* mark any of these pages as "dma-pinned"? In other words,
why even have a separate gup/pup API? And in fact, perhaps eventually
we'll just get rid of the get_user_pages*() side of it. But the pin
count will need to remain, in order to discern between DMA pins and
temporary refcount boosts.

thanks,
-- 
John Hubbard
NVIDIA


  reply	other threads:[~2020-09-21 22:18 UTC|newest]

Thread overview: 110+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-09-21 21:17 [PATCH 0/5] mm: Break COW for pinned pages during fork() Peter Xu
2020-09-21 21:17 ` [PATCH 1/5] mm: Introduce mm_struct.has_pinned Peter Xu
2020-09-21 21:43   ` Jann Horn
2020-09-21 22:30     ` Peter Xu
2020-09-21 22:47       ` Jann Horn
2020-09-22 11:54         ` Jason Gunthorpe
2020-09-22 14:28           ` Peter Xu
2020-09-22 15:56             ` Jason Gunthorpe
2020-09-22 16:25               ` Linus Torvalds
2020-09-21 23:53   ` John Hubbard
2020-09-22  0:01     ` John Hubbard
2020-09-22 15:17     ` Peter Xu
2020-09-22 16:10       ` Jason Gunthorpe
2020-09-22 17:54         ` Peter Xu
2020-09-22 19:11           ` Jason Gunthorpe
2020-09-23  0:27             ` Peter Xu
2020-09-23 13:10               ` Peter Xu
2020-09-23 14:20                 ` Jan Kara
2020-09-23 17:12                   ` Jason Gunthorpe
2020-09-24  7:44                     ` Jan Kara
2020-09-24 14:02                       ` Jason Gunthorpe
2020-09-24 14:45                         ` Jan Kara
2020-09-23 17:07               ` Jason Gunthorpe
2020-09-24 14:35                 ` Peter Xu
2020-09-24 16:51                   ` Jason Gunthorpe
2020-09-24 17:55                     ` Peter Xu
2020-09-24 18:15                       ` Jason Gunthorpe
2020-09-24 18:34                         ` Peter Xu
2020-09-24 18:39                           ` Jason Gunthorpe
2020-09-24 21:30                             ` Peter Xu
2020-09-25 19:56                               ` Linus Torvalds
2020-09-25 21:06                                 ` Linus Torvalds
2020-09-26  0:41                                   ` Jason Gunthorpe
2020-09-26  1:15                                     ` Linus Torvalds
2020-09-26 22:28                                       ` Linus Torvalds
2020-09-27  6:23                                         ` Leon Romanovsky
2020-09-27 18:16                                           ` Linus Torvalds
2020-09-27 18:45                                             ` Linus Torvalds
2020-09-28 12:49                                               ` Jason Gunthorpe
2020-09-28 16:17                                                 ` Linus Torvalds
2020-09-28 17:22                                                   ` Peter Xu
2020-09-28 17:54                                                     ` Linus Torvalds
2020-09-28 18:39                                                       ` Jason Gunthorpe
2020-09-28 19:29                                                         ` Linus Torvalds
2020-09-28 23:57                                                           ` Jason Gunthorpe
2020-09-29  0:18                                                             ` John Hubbard
2020-09-28 19:36                                                         ` Linus Torvalds
2020-09-28 19:50                                                           ` Linus Torvalds
2020-09-28 22:51                                                             ` Jason Gunthorpe
2020-09-29  0:30                                                               ` Peter Xu
2020-10-08  5:49                                                             ` Leon Romanovsky
2020-09-28 17:13                                             ` Peter Xu
2020-09-25 21:13                                 ` Peter Xu
2020-09-25 22:08                                   ` Linus Torvalds
2020-09-22 18:02       ` John Hubbard
2020-09-22 18:15         ` Peter Xu
2020-09-22 19:11       ` John Hubbard
2020-09-27  0:41   ` [mm] 698ac7610f: will-it-scale.per_thread_ops 8.2% improvement kernel test robot
2020-09-21 21:17 ` [PATCH 2/5] mm/fork: Pass new vma pointer into copy_page_range() Peter Xu
2020-09-21 21:17 ` [PATCH 3/5] mm: Rework return value for copy_one_pte() Peter Xu
2020-09-22  7:11   ` John Hubbard
2020-09-22 15:29     ` Peter Xu
2020-09-22 10:08   ` Oleg Nesterov
2020-09-22 10:18     ` Oleg Nesterov
2020-09-22 15:36       ` Peter Xu
2020-09-22 15:48         ` Oleg Nesterov
2020-09-22 16:03           ` Peter Xu
2020-09-22 16:53             ` Oleg Nesterov
2020-09-22 18:13               ` Peter Xu
2020-09-22 18:23                 ` Oleg Nesterov
2020-09-22 18:49                   ` Peter Xu
2020-09-23  6:52                     ` Oleg Nesterov
2020-09-23 17:16   ` Linus Torvalds
2020-09-23 21:24     ` Linus Torvalds
2020-09-21 21:20 ` [PATCH 4/5] mm: Do early cow for pinned pages during fork() for ptes Peter Xu
2020-09-21 21:55   ` Jann Horn
2020-09-21 22:18     ` John Hubbard [this message]
2020-09-21 22:27       ` Jann Horn
2020-09-22  0:08         ` John Hubbard
2020-09-21 22:27     ` Peter Xu
2020-09-22 11:48   ` Oleg Nesterov
2020-09-22 12:40     ` Oleg Nesterov
2020-09-22 15:58       ` Peter Xu
2020-09-22 16:52         ` Oleg Nesterov
2020-09-22 18:34           ` Peter Xu
2020-09-22 18:44             ` Oleg Nesterov
2020-09-23  1:03               ` Peter Xu
2020-09-23 20:25                 ` Linus Torvalds
2020-09-24 15:08                   ` Peter Xu
2020-09-24 11:48   ` Kirill Tkhai
2020-09-24 15:16     ` Peter Xu
2020-09-21 21:20 ` [PATCH 5/5] mm/thp: Split huge pmds/puds if they're pinned when fork() Peter Xu
2020-09-22  6:41   ` John Hubbard
2020-09-22 10:33     ` Jan Kara
2020-09-22 20:01       ` John Hubbard
2020-09-23  9:22         ` Jan Kara
2020-09-23 13:50           ` Peter Xu
2020-09-23 14:01             ` Jan Kara
2020-09-23 15:44               ` Peter Xu
2020-09-23 20:19                 ` John Hubbard
2020-09-24 18:49                   ` Peter Xu
2020-09-23 16:06     ` Peter Xu
2020-09-22 12:05   ` Jason Gunthorpe
2020-09-23 15:24     ` Peter Xu
2020-09-23 16:07       ` Yang Shi
2020-09-24 15:47         ` Peter Xu
2020-09-24 17:29           ` Yang Shi
2020-09-23 17:17       ` Jason Gunthorpe
2020-09-23 10:21 ` [PATCH 0/5] mm: Break COW for pinned pages during fork() Leon Romanovsky
2020-09-23 15:37   ` Peter Xu

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