All of lore.kernel.org
 help / color / mirror / Atom feed
From: Linus Torvalds <torvalds@linux-foundation.org>
To: Nadav Amit <nadav.amit@gmail.com>
Cc: John Hubbard <jhubbard@nvidia.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Jann Horn <jannh@google.com>, X86 ML <x86@kernel.org>,
	Matthew Wilcox <willy@infradead.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	kernel list <linux-kernel@vger.kernel.org>,
	Linux-MM <linux-mm@kvack.org>,
	Andrea Arcangeli <aarcange@redhat.com>,
	"Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
	jroedel@suse.de, ubizjak@gmail.com,
	Alistair Popple <apopple@nvidia.com>
Subject: Re: [PATCH 01/13] mm: Update ptep_get_lockless()s comment
Date: Sat, 29 Oct 2022 14:12:15 -0700	[thread overview]
Message-ID: <CAHk-=wj-SqWifNOnZ49zvGP9h06mJ36PDWkcMbfjnt=2gvE+ig@mail.gmail.com> (raw)
In-Reply-To: <B39368D9-7E5D-4864-819C-CDEEAB4BD92D@gmail.com>

On Sat, Oct 29, 2022 at 1:56 PM Nadav Amit <nadav.amit@gmail.com> wrote:
>
> On Oct 29, 2022, at 1:15 PM, Linus Torvalds <torvalds@linux-foundation.org> wrote:
>
> > (b) we could move the "page_remove_rmap()" into the "flush-and-free" path too
> >
> > And (b) would be fairly easy - same model as that dirty bit patch,
> > just a 'do page_remove_rmap too' - except page_remove_rmap() wants the
> > vma as well (and we delay the TLB flush over multiple vma's, so it's
> > not just a "save vma in mmu_gather”).
>
> (b) sounds reasonable and may potentially allow future performance
> improvements (batching, doing stuff without locks).

So the thing is, I think (b) makes sense from a TLB flush standpoint,
but as mentioned, if filesystems _really_ want to feel in control,
none of these locks fundamentally help, because the whole "gup +
set_page_dirty()" situation still exists.

And that fundamentally does not hold any locks between the lookup and
the set_page_dirty(), and fundamentally doesn't stop an rmap from
happening in between, so while the zap_page_range() and dirty TLB case
case be dealt with that way, you don't actually fix any fundamental
issues.

Now, neither does my (c) case on its own, but as John Hubbard alluded
to, *if* we had some sane serialization for a struct page, maybe that
could then be used to at least avoid the issue with "rmap no longer
exists", and make the filesystem handling case a bit better.

IOW, I really do think that not only is (a) the current solution, it's
the *correct* solution.

But it is possible that (c) could then be used as a way to make (a)
more palatable to filesystems, in that at least then there would be
some way to serialize with "set_page_dirty()".

But (b) cannot be used for that - because GUP fundamentally breaks
that rmap association.

It's all nasty.

                Linus

  parent reply	other threads:[~2022-10-29 21:12 UTC|newest]

Thread overview: 148+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-10-22 11:14 [PATCH 00/13] Clean up pmd_get_atomic() and i386-PAE Peter Zijlstra
2022-10-22 11:14 ` [PATCH 01/13] mm: Update ptep_get_lockless()s comment Peter Zijlstra
2022-10-24  5:42   ` John Hubbard
2022-10-24  8:00     ` Peter Zijlstra
2022-10-24 19:58       ` Jann Horn
2022-10-24 20:19         ` Linus Torvalds
2022-10-24 20:23           ` Jann Horn
2022-10-24 20:36             ` Linus Torvalds
2022-10-25  3:21             ` Matthew Wilcox
2022-10-25  7:54               ` Alistair Popple
2022-10-25 13:33                 ` Peter Zijlstra
2022-10-25 13:44                 ` Jann Horn
2022-10-26  0:45                   ` Alistair Popple
2022-10-25 14:02         ` Peter Zijlstra
2022-10-25 14:18           ` Jann Horn
2022-10-25 15:06             ` Peter Zijlstra
2022-10-26 16:45               ` Jann Horn
2022-10-27  7:08                 ` Peter Zijlstra
2022-10-27 18:13                   ` Linus Torvalds
2022-10-27 19:35                     ` Peter Zijlstra
2022-10-27 19:43                       ` Linus Torvalds
2022-10-27 20:15                     ` Nadav Amit
2022-10-27 20:31                       ` Linus Torvalds
2022-10-27 21:44                         ` Nadav Amit
2022-10-28 23:57                           ` Nadav Amit
2022-10-29  0:42                             ` Linus Torvalds
2022-10-29 18:05                               ` Nadav Amit
2022-10-29 18:36                                 ` Linus Torvalds
2022-10-29 18:58                                   ` Linus Torvalds
2022-10-29 19:14                                     ` Linus Torvalds
2022-10-29 19:28                                       ` Nadav Amit
2022-10-30  0:18                                       ` Nadav Amit
2022-10-30  2:17                                     ` Nadav Amit
2022-10-30 18:19                                       ` Linus Torvalds
2022-10-30 18:51                                         ` Linus Torvalds
2022-10-30 22:47                                           ` Linus Torvalds
2022-10-31  1:47                                             ` Linus Torvalds
2022-10-31  4:09                                               ` Nadav Amit
2022-10-31  4:55                                                 ` Nadav Amit
2022-10-31  5:00                                                 ` Linus Torvalds
2022-10-31 15:43                                                   ` Nadav Amit
2022-10-31 17:32                                                     ` Linus Torvalds
2022-10-31  9:36                                               ` Peter Zijlstra
2022-10-31 17:28                                                 ` Linus Torvalds
2022-10-31 18:43                                                   ` mm: delay rmap removal until after TLB flush Linus Torvalds
2022-11-02  9:14                                                     ` Christian Borntraeger
2022-11-02  9:23                                                       ` Christian Borntraeger
2022-11-02 17:55                                                       ` Linus Torvalds
2022-11-02 18:28                                                         ` Linus Torvalds
2022-11-02 22:29                                                         ` Gerald Schaefer
2022-11-02 12:45                                                     ` Peter Zijlstra
2022-11-02 22:31                                                     ` Gerald Schaefer
2022-11-02 23:13                                                       ` Linus Torvalds
2022-11-03  9:52                                                     ` David Hildenbrand
2022-11-03 16:54                                                       ` Linus Torvalds
2022-11-03 17:09                                                         ` Linus Torvalds
2022-11-03 17:36                                                           ` David Hildenbrand
2022-11-04  6:33                                                     ` Alexander Gordeev
2022-11-04 17:35                                                       ` Linus Torvalds
2022-11-06 21:06                                                         ` Hugh Dickins
2022-11-06 22:34                                                           ` Linus Torvalds
2022-11-06 23:14                                                             ` Andrew Morton
2022-11-07  0:06                                                               ` Stephen Rothwell
2022-11-07 16:19                                                               ` Linus Torvalds
2022-11-07 23:02                                                                 ` Andrew Morton
2022-11-07 23:44                                                                   ` Stephen Rothwell
2022-11-07  9:12                                                           ` Peter Zijlstra
2022-11-07 20:07                                                           ` Johannes Weiner
2022-11-07 20:29                                                             ` Linus Torvalds
2022-11-07 23:47                                                               ` Linus Torvalds
2022-11-08  4:28                                                                 ` Linus Torvalds
2022-11-08 19:56                                                                   ` Linus Torvalds
2022-11-08 20:03                                                                     ` Konstantin Ryabitsev
2022-11-08 20:18                                                                       ` Linus Torvalds
2022-11-08 19:41                                                                 ` [PATCH 1/4] mm: introduce 'encoded' page pointers with embedded extra bits Linus Torvalds
2022-11-08 20:37                                                                   ` Nadav Amit
2022-11-08 20:46                                                                     ` Linus Torvalds
2022-11-09  6:36                                                                   ` Alexander Gordeev
2022-11-09 18:00                                                                     ` Linus Torvalds
2022-11-09 20:02                                                                       ` Linus Torvalds
2022-11-08 19:41                                                                 ` [PATCH 2/4] mm: teach release_pages() to take an array of encoded page pointers too Linus Torvalds
2022-11-08 19:41                                                                 ` [PATCH 3/4] mm: mmu_gather: prepare to gather encoded page pointers with flags Linus Torvalds
2022-11-08 19:41                                                                 ` [PATCH 4/4] mm: delay page_remove_rmap() until after the TLB has been flushed Linus Torvalds
2022-11-08 20:48                                                                   ` [lkp] [+115 bytes kernel size regression] [i386-tinyconfig] [0309f16088] " kernel test robot
2022-11-08 21:01                                                                     ` Linus Torvalds
2022-11-08 21:05                                                                   ` [PATCH 4/4] " Nadav Amit
2022-11-09 15:53                                                                   ` Johannes Weiner
2022-11-09 19:31                                                                     ` Hugh Dickins
2022-10-31  9:39                                               ` [PATCH 01/13] mm: Update ptep_get_lockless()s comment Peter Zijlstra
2022-10-31 17:22                                                 ` Linus Torvalds
2022-10-31  9:46                                               ` Peter Zijlstra
2022-10-31  9:28                                             ` Peter Zijlstra
2022-10-31 17:19                                               ` Linus Torvalds
2022-10-30 19:34                                         ` Nadav Amit
2022-10-29 19:39                                   ` John Hubbard
2022-10-29 20:15                                     ` Linus Torvalds
2022-10-29 20:30                                       ` Linus Torvalds
2022-10-29 20:42                                         ` John Hubbard
2022-10-29 20:56                                       ` Nadav Amit
2022-10-29 21:03                                         ` Nadav Amit
2022-10-29 21:12                                         ` Linus Torvalds [this message]
2022-10-29 20:59                                       ` Theodore Ts'o
2022-10-26 19:43               ` Nadav Amit
2022-10-27  7:27                 ` Peter Zijlstra
2022-10-27 17:30                   ` Nadav Amit
2022-10-22 11:14 ` [PATCH 02/13] x86/mm/pae: Make pmd_t similar to pte_t Peter Zijlstra
2022-10-22 11:14 ` [PATCH 03/13] sh/mm: " Peter Zijlstra
2022-12-21 13:54   ` Guenter Roeck
2022-10-22 11:14 ` [PATCH 04/13] mm: Fix pmd_read_atomic() Peter Zijlstra
2022-10-22 17:30   ` Linus Torvalds
2022-10-24  8:09     ` Peter Zijlstra
2022-11-01 12:41     ` Peter Zijlstra
2022-11-01 17:42       ` Linus Torvalds
2022-11-02  9:12       ` [tip: x86/mm] mm: Convert __HAVE_ARCH_P..P_GET to the new style tip-bot2 for Peter Zijlstra
2022-11-03 21:15       ` tip-bot2 for Peter Zijlstra
2022-12-17 18:55       ` tip-bot2 for Peter Zijlstra
2022-10-22 11:14 ` [PATCH 05/13] mm: Rename GUP_GET_PTE_LOW_HIGH Peter Zijlstra
2022-10-22 11:14 ` [PATCH 06/13] mm: Rename pmd_read_atomic() Peter Zijlstra
2022-10-22 11:14 ` [PATCH 07/13] mm/gup: Fix the lockless PMD access Peter Zijlstra
2022-10-23  0:42   ` Hugh Dickins
2022-10-24  7:42     ` Peter Zijlstra
2022-10-25  3:58       ` Hugh Dickins
2022-10-22 11:14 ` [PATCH 08/13] x86/mm/pae: Dont (ab)use atomic64 Peter Zijlstra
2022-10-22 11:14 ` [PATCH 09/13] x86/mm/pae: Use WRITE_ONCE() Peter Zijlstra
2022-10-22 17:42   ` Linus Torvalds
2022-10-24 10:21     ` Peter Zijlstra
2022-10-22 11:14 ` [PATCH 10/13] x86/mm/pae: Be consistent with pXXp_get_and_clear() Peter Zijlstra
2022-10-22 17:53   ` Linus Torvalds
2022-10-24 11:13     ` Peter Zijlstra
2022-10-22 11:14 ` [PATCH 11/13] x86_64: Remove pointless set_64bit() usage Peter Zijlstra
2022-10-22 17:55   ` Linus Torvalds
2022-11-03 19:09   ` Nathan Chancellor
2022-11-03 19:23     ` Uros Bizjak
2022-11-03 19:35       ` Nathan Chancellor
2022-11-03 20:39         ` Linus Torvalds
2022-11-03 21:06           ` Peter Zijlstra
2022-11-04 16:01           ` Peter Zijlstra
2022-11-04 17:15             ` Linus Torvalds
2022-11-05 13:29               ` Jason A. Donenfeld
2022-11-05 15:14                 ` Peter Zijlstra
2022-11-05 20:54                   ` Jason A. Donenfeld
2022-11-07  9:14                   ` David Laight
2022-12-19 15:44               ` Peter Zijlstra
2022-10-22 11:14 ` [PATCH 12/13] x86/mm/pae: Get rid of set_64bit() Peter Zijlstra
2022-10-22 11:14 ` [PATCH 13/13] mm: Remove pointless barrier() after pmdp_get_lockless() Peter Zijlstra
2022-10-22 19:59   ` Yu Zhao
2022-10-22 17:57 ` [PATCH 00/13] Clean up pmd_get_atomic() and i386-PAE Linus Torvalds
2022-10-29 12:21 ` Peter Zijlstra

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to='CAHk-=wj-SqWifNOnZ49zvGP9h06mJ36PDWkcMbfjnt=2gvE+ig@mail.gmail.com' \
    --to=torvalds@linux-foundation.org \
    --cc=aarcange@redhat.com \
    --cc=akpm@linux-foundation.org \
    --cc=apopple@nvidia.com \
    --cc=jannh@google.com \
    --cc=jhubbard@nvidia.com \
    --cc=jroedel@suse.de \
    --cc=kirill.shutemov@linux.intel.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-mm@kvack.org \
    --cc=nadav.amit@gmail.com \
    --cc=peterz@infradead.org \
    --cc=ubizjak@gmail.com \
    --cc=willy@infradead.org \
    --cc=x86@kernel.org \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body.
This is an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.