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
next prev parent reply other threads:[~2020-09-21 22:18 UTC|newest]
Thread overview: 133+ 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 21:43 ` Jann Horn
2020-09-21 22:30 ` Peter Xu
2020-09-21 22:47 ` Jann Horn
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
2020-09-22 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 19:56 ` Linus Torvalds
2020-09-25 21:06 ` 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 1:15 ` Linus Torvalds
2020-09-26 22:28 ` 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 18:16 ` Linus Torvalds
2020-09-27 18:45 ` 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 16:17 ` Linus Torvalds
2020-09-28 17:22 ` Peter Xu
2020-09-28 17:54 ` Linus Torvalds
2020-09-28 17:54 ` Linus Torvalds
2020-09-28 18:39 ` Jason Gunthorpe
2020-09-28 19:29 ` Linus Torvalds
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 19:36 ` Linus Torvalds
2020-09-28 19:50 ` 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
2020-09-25 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
2020-09-27 0:41 ` 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 17:16 ` Linus Torvalds
2020-09-23 21:24 ` 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 21:55 ` Jann Horn
2020-09-21 22:18 ` John Hubbard [this message]
2020-09-21 22:27 ` Jann Horn
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
2020-09-23 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
2020-09-23 16:07 ` Yang Shi
2020-09-24 15:47 ` Peter Xu
2020-09-24 17:29 ` Yang Shi
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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