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From: Nadav Amit <nadav.amit@gmail.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Jann Horn <jannh@google.com>, John Hubbard <jhubbard@nvidia.com>,
	X86 ML <x86@kernel.org>, Matthew Wilcox <willy@infradead.org>,
	Linus Torvalds <torvalds@linux-foundation.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
Subject: Re: [PATCH 01/13] mm: Update ptep_get_lockless()s comment
Date: Wed, 26 Oct 2022 22:43:21 +0300	[thread overview]
Message-ID: <A3677D1D-3631-4E57-8C5C-6C202B47700C@gmail.com> (raw)
In-Reply-To: <Y1f7YvKuwOl1XEwU@hirez.programming.kicks-ass.net>

On Oct 25, 2022, at 6:06 PM, Peter Zijlstra <peterz@infradead.org> wrote:

> 			if (!force_flush && !tlb->fullmm && details &&
> +			    details->zap_flags & ZAP_FLAG_FORCE_FLUSH)
> +				force_flush = 1;

Isn’t it too big of a hammer?

At the same time, the whole reasoning about TLB flushes is not getting any
simpler. We had cases in which MADV_DONTNEED and another concurrent
operation that effectively zapped PTEs (e.g., another MADV_DONTNEED) caused
the zap_pte_range() to skip entries since pte_none() was true. To resolve
these cases we relied on tlb_finish_mmu() to flush the range when needed
(i.e., flush the whole range when mm_tlb_flush_nested()).

Now, I do not have a specific broken scenario in mind following this change,
but it is all sounds to me a bit dangerous and at same time can potentially
introduce new overheads.

One alternative may be using mm_tlb_flush_pending() when setting a new PTE
to check for pending flushes and flushing the TLB if that is the case. This
is somewhat similar to what ptep_clear_flush() does. Anyhow, I guess this
might induce some overheads. As noted before, it is possible to track
pending TLB flushes in VMA/page-table granularity, with different tradeoffs
of overheads.


  parent reply	other threads:[~2022-10-26 19:44 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
2022-10-29 20:59                                       ` Theodore Ts'o
2022-10-26 19:43               ` Nadav Amit [this message]
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

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