From: Jan Kara <jack@suse.cz>
To: John Hubbard <jhubbard@nvidia.com>
Cc: Jan Kara <jack@suse.cz>, Peter Xu <peterx@redhat.com>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org,
Linus Torvalds <torvalds@linux-foundation.org>,
Michal Hocko <mhocko@suse.com>,
Kirill Shutemov <kirill@shutemov.name>,
Jann Horn <jannh@google.com>, Oleg Nesterov <oleg@redhat.com>,
Kirill Tkhai <ktkhai@virtuozzo.com>,
Hugh Dickins <hughd@google.com>,
Leon Romanovsky <leonro@nvidia.com>,
Christoph Hellwig <hch@lst.de>,
Andrew Morton <akpm@linux-foundation.org>,
Jason Gunthorpe <jgg@ziepe.ca>,
Andrea Arcangeli <aarcange@redhat.com>
Subject: Re: [PATCH 5/5] mm/thp: Split huge pmds/puds if they're pinned when fork()
Date: Wed, 23 Sep 2020 11:22:05 +0200 [thread overview]
Message-ID: <20200923092205.GA6719@quack2.suse.cz> (raw)
In-Reply-To: <4a65586e-9282-beb0-1880-1ef8da03727c@nvidia.com>
On Tue 22-09-20 13:01:13, John Hubbard wrote:
> On 9/22/20 3:33 AM, Jan Kara wrote:
> > On Mon 21-09-20 23:41:16, John Hubbard wrote:
> > > On 9/21/20 2:20 PM, Peter Xu wrote:
> > > ...
> > > > + if (unlikely(READ_ONCE(src_mm->has_pinned) &&
> > > > + page_maybe_dma_pinned(src_page))) {
> > >
> > > This condition would make a good static inline function. It's used in 3
> > > places, and the condition is quite special and worth documenting, and
> > > having a separate function helps with that, because the function name
> > > adds to the story. I'd suggest approximately:
> > >
> > > page_likely_dma_pinned()
> > >
> > > for the name.
> >
> > Well, but we should also capture that this really only works for anonymous
> > pages. For file pages mm->has_pinned does not work because the page may be
> > still pinned by completely unrelated process as Jann already properly
> > pointed out earlier in the thread. So maybe anon_page_likely_pinned()?
> > Possibly also assert PageAnon(page) in it if we want to be paranoid...
> >
> > Honza
>
> The file-backed case doesn't really change anything, though:
> page_maybe_dma_pinned() is already a "fuzzy yes" in the same sense: you
> can get a false positive. Just like here, with an mm->has_pinned that
> could be a false positive for a process.
>
> And for that reason, I'm also not sure an "assert PageAnon(page)" is
> desirable. That assertion would prevent file-backed callers from being
> able to call a function that provides a fuzzy answer, but I don't see
> why you'd want or need to do that. The goal here is to make the fuzzy
> answer a little bit more definite, but it's not "broken" just because
> the result is still fuzzy, right?
>
> Apologies if I'm missing a huge point here... :)
But the problem is that if you apply mm->has_pinned check on file pages,
you can get false negatives now. And that's not acceptable...
Honza
--
Jan Kara <jack@suse.com>
SUSE Labs, CR
next prev parent reply other threads:[~2020-09-23 9:22 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
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 [this message]
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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