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From: Linus Torvalds <torvalds@linux-foundation.org>
To: Yang Shi <yang.shi@linux.alibaba.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	 Johannes Weiner <hannes@cmpxchg.org>,
	Hillf Danton <hdanton@sina.com>, Hugh Dickins <hughd@google.com>,
	 Josef Bacik <josef@toxicpanda.com>,
	 "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
	Linux-MM <linux-mm@kvack.org>,
	 mm-commits@vger.kernel.org, Will Deacon <will.deacon@arm.com>,
	 Matthew Wilcox <willy@infradead.org>,
	Yu Xu <xuyu@linux.alibaba.com>
Subject: Re: [patch 01/15] mm/memory.c: avoid access flag update TLB flush for retried page fault
Date: Mon, 27 Jul 2020 11:04:55 -0700
Message-ID: <CAHk-=wjve+T9+-nTsxssKQL__mucv_SpLoq1C0xv5NB_n0+wqQ@mail.gmail.com> (raw)
In-Reply-To: <45015c63-e719-a58a-7d07-6c156273a890@linux.alibaba.com>

On Mon, Jul 27, 2020 at 10:52 AM Yang Shi <yang.shi@linux.alibaba.com> wrote:
>
> It looks normal page is skipped too unless it is a write fault. The
> comment might be a little bit misleading.

No the comment is fine - in that it matches the code.

It's the code _and_ the comment that I find to be garbage.

> Read fault should just change young bit and typically TLB won't get
> flushed if just young bit is changed and TLB flush can be deferred again
> to write fault which may change access permission and/or dirty bit.

This is the part I disagree with.

A read fault could easily cause the exact same issue, exactly because
people do young bits in software too.

It's just harder to trigger, because the young bit is typically set
initially - in ways that the dirty bit easily isn't.

So to get to the "on, young bit wasn't set, the TLB has the 'fault on
access' bit set, *and* we raced on two different CPU's at the same
time" condition is much *much* harder than the write bit is.

But it seems to be no different in theory.

So I think the whole "treat write/dirty specially" thing is complete
garbage. Sure, it speeds things up. But it speeds things up by being
wrong.

                  Linus


  reply index

Thread overview: 69+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-24  4:14 incoming Andrew Morton
2020-07-24  4:15 ` [patch 01/15] mm/memory.c: avoid access flag update TLB flush for retried page fault Andrew Morton
2020-07-24  4:38   ` Yang Shi
2020-07-24  4:56     ` Andrew Morton
2020-07-24 19:27   ` Linus Torvalds
2020-07-24 20:22     ` Linus Torvalds
2020-07-25  0:36       ` Yang Shi
2020-07-25  1:29         ` Linus Torvalds
2020-07-25 15:58           ` Catalin Marinas
2020-07-28  9:22             ` Will Deacon
2020-07-28  9:39               ` Catalin Marinas
2020-07-28 10:07                 ` Yu Xu
2020-07-28 11:46                   ` Catalin Marinas
2020-07-28 10:21                 ` Will Deacon
2020-07-28 18:28                 ` Linus Torvalds
2020-07-27 17:52           ` Yang Shi
2020-07-27 18:04             ` Linus Torvalds [this message]
2020-07-27 18:42               ` Catalin Marinas
2020-07-27 20:56                 ` Linus Torvalds
2020-07-27 22:34               ` Yang Shi
2020-07-27  7:31       ` Yu Xu
2020-07-27 11:05         ` Catalin Marinas
2020-07-27 17:01           ` Linus Torvalds
2020-07-28 11:19             ` Catalin Marinas
2020-07-27 17:12           ` Yu Xu
2020-07-27 18:04             ` Yang Shi
2020-07-27 18:37               ` Linus Torvalds
2020-07-27 22:43                 ` Yang Shi
2020-07-28  0:38                   ` Linus Torvalds
2020-07-28  0:13                 ` Yu Xu
2020-07-28 10:53                 ` Nicholas Piggin
2020-07-28 19:02                   ` Linus Torvalds
2020-07-28 22:53                     ` Nicholas Piggin
2020-07-29 13:58                       ` Michael Ellerman
2020-07-28  6:41             ` Yu Xu
2020-07-24  4:15 ` [patch 02/15] mm/mmap.c: close race between munmap() and expand_upwards()/downwards() Andrew Morton
2020-07-24  4:15 ` [patch 03/15] vfs/xattr: mm/shmem: kernfs: release simple xattr entry in a right way Andrew Morton
2020-07-24  4:15 ` [patch 04/15] mm: initialize return of vm_insert_pages Andrew Morton
2020-07-24  4:15 ` [patch 05/15] mm/memcontrol: fix OOPS inside mem_cgroup_get_nr_swap_pages() Andrew Morton
2020-07-24  4:15 ` [patch 06/15] mm/memcg: fix refcount error while moving and swapping Andrew Morton
2020-07-24 13:41   ` Alex Shi
2020-07-24  4:15 ` [patch 07/15] mm: memcg/slab: fix memory leak at non-root kmem_cache destroy Andrew Morton
2020-07-24  4:15 ` [patch 08/15] mm/hugetlb: avoid hardcoding while checking if cma is enabled Andrew Morton
2020-07-24  4:15 ` [patch 09/15] khugepaged: fix null-pointer dereference due to race Andrew Morton
2020-07-24  4:15 ` [patch 10/15] mailmap: add entry for Mike Rapoport Andrew Morton
2020-07-24  4:15 ` [patch 11/15] squashfs: fix length field overlap check in metadata reading Andrew Morton
2020-07-24  4:15 ` [patch 12/15] scripts/decode_stacktrace: strip basepath from all paths Andrew Morton
2020-07-24  4:15 ` [patch 13/15] io-mapping: indicate mapping failure Andrew Morton
2020-07-24  4:15 ` [patch 14/15] MAINTAINERS: add KCOV section Andrew Morton
2020-07-24  4:15 ` [patch 15/15] scripts/gdb: fix lx-symbols 'gdb.error' while loading modules Andrew Morton
2020-07-28  1:19 ` mmotm 2020-07-27-18-18 uploaded Andrew Morton
2020-07-28  2:14   ` Stephen Rothwell
2020-07-28  3:22   ` mmotm 2020-07-27-18-18 uploaded (drivers/scsi/ufs/: SCSI_UFS_EXYNOS) Randy Dunlap
2020-07-28  8:23     ` Alim Akhtar
2020-07-28 12:33   ` mmotm 2020-07-27-18-18 uploaded (mm/page_alloc.c) Randy Dunlap
2020-07-28 21:55     ` Andrew Morton
2020-07-28 22:20       ` Stephen Rothwell
2020-07-28 22:31         ` Andrew Morton
2020-07-29 14:18           ` Michael S. Tsirkin
2020-07-29 14:38             ` David Hildenbrand
2020-07-29 16:14               ` David Hildenbrand
2020-07-29 17:29                 ` Randy Dunlap
2020-07-28 22:39       ` Randy Dunlap
2020-07-29  1:43         ` Nathan Chancellor
2020-07-29  1:44         ` Andrew Morton
2020-07-29  2:04           ` Randy Dunlap
2020-07-29 14:09           ` make oldconfig (Re: mmotm 2020-07-27-18-18 uploaded (mm/page_alloc.c)) Alexey Dobriyan
2020-07-31 23:46 ` mmotm 2020-07-31-16-45 uploaded Andrew Morton
2020-08-01  5:24   ` mmotm 2020-07-31-16-45 uploaded (drivers/staging/vc04_services/) Randy Dunlap

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