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From: Suren Baghdasaryan <surenb@google.com>
To: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Cc: akpm@linux-foundation.org, michel@lespinasse.org,
	jglisse@google.com, mhocko@suse.com, vbabka@suse.cz,
	hannes@cmpxchg.org, mgorman@techsingularity.net,
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Subject: Re: [PATCH v3 16/35] mm/mmap: write-lock VMAs before merging, splitting or expanding them
Date: Thu, 23 Feb 2023 09:46:06 -0800	[thread overview]
Message-ID: <CAJuCfpFmQYJ-0jFfO1YW7vgZ_DAm-ftrWfTm2AtFQ5u=AGgwVg@mail.gmail.com> (raw)
In-Reply-To: <Y/d9b5Ev6vAEzzVO@localhost>

On Thu, Feb 23, 2023 at 6:51 AM Hyeonggon Yoo <42.hyeyoo@gmail.com> wrote:
>
> On Wed, Feb 15, 2023 at 09:17:31PM -0800, Suren Baghdasaryan wrote:
> > Decisions about whether VMAs can be merged, split or expanded must be
> > made while VMAs are protected from the changes which can affect that
> > decision. For example, merge_vma uses vma->anon_vma in its decision
>
> Did you mean vma_merge()?

Correct.

>
> > whether the VMA can be merged. Meanwhile, page fault handler changes
> > vma->anon_vma during COW operation.
> > Write-lock all VMAs which might be affected by a merge or split operation
> > before making decision how such operations should be performed.
> >
>
> It doesn't make sense (to me) to update vma->anon_vma during COW fault.
>
> AFAIK children's vma->anon_vma is allocated during fork() and
> page->anon_vma is updated on COW to reduce rmap walking because it's now
> unshared, no?
>
> As patch 26 just falls back to mmap_lock if no anon_vma is set,
> I think we can assume nothing updates vma->anon_vma (and its interval
> tree) if we are holding mmap_lock in write mode.
>
> Or am I missing something?

No, I think you are right. Patch 26 was added in the later versions of
this patchset and at the time this patch was written vma->anon_vma
could change during page fault handling under only per-VMA lock
protection.
So, this patch can be simplified. We still want to prevent concurrent
page faults while VMA is being merged or split (simply because par-VMA
lock that page fault handler holds might become the wrong one if the
VMA is split or merged from under it) but the timing of taking per-VMA
lock does not have to be *before* can_vma_merge_{before|after}. Does
that make sense?

>
> --
> Regards,
> Hyeonggon
>
> --
> To unsubscribe from this group and stop receiving emails from it, send an email to kernel-team+unsubscribe@android.com.
>

  parent reply	other threads:[~2023-02-23 17:46 UTC|newest]

Thread overview: 67+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-02-16  5:17 [PATCH v3 00/35] Per-VMA locks Suren Baghdasaryan
2023-02-16  5:17 ` [PATCH v3 01/35] maple_tree: Be more cautious about dead nodes Suren Baghdasaryan
2023-02-16  5:17 ` [PATCH v3 02/35] maple_tree: Detect dead nodes in mas_start() Suren Baghdasaryan
2023-02-16  5:17 ` [PATCH v3 03/35] maple_tree: Fix freeing of nodes in rcu mode Suren Baghdasaryan
2023-02-16  5:17 ` [PATCH v3 04/35] maple_tree: remove extra smp_wmb() from mas_dead_leaves() Suren Baghdasaryan
2023-02-16  5:17 ` [PATCH v3 05/35] maple_tree: Fix write memory barrier of nodes once dead for RCU mode Suren Baghdasaryan
2023-02-16  5:17 ` [PATCH v3 06/35] maple_tree: Add smp_rmb() to dead node detection Suren Baghdasaryan
2023-02-16  5:17 ` [PATCH v3 07/35] maple_tree: Add RCU lock checking to rcu callback functions Suren Baghdasaryan
2023-02-16  5:17 ` [PATCH v3 08/35] mm: Enable maple tree RCU mode by default Suren Baghdasaryan
2023-02-16  5:17 ` [PATCH v3 09/35] mm: introduce CONFIG_PER_VMA_LOCK Suren Baghdasaryan
2023-02-16  5:17 ` [PATCH v3 10/35] mm: rcu safe VMA freeing Suren Baghdasaryan
2023-02-16  5:17 ` [PATCH v3 11/35] mm: move mmap_lock assert function definitions Suren Baghdasaryan
2023-02-16  5:17 ` [PATCH v3 12/35] mm: add per-VMA lock and helper functions to control it Suren Baghdasaryan
2023-02-16  5:17 ` [PATCH v3 13/35] mm: mark VMA as being written when changing vm_flags Suren Baghdasaryan
2023-02-16  5:17 ` [PATCH v3 14/35] mm/mmap: move VMA locking before vma_adjust_trans_huge call Suren Baghdasaryan
2023-02-16  5:17 ` [PATCH v3 15/35] mm/khugepaged: write-lock VMA while collapsing a huge page Suren Baghdasaryan
2023-02-16  5:17 ` [PATCH v3 16/35] mm/mmap: write-lock VMAs before merging, splitting or expanding them Suren Baghdasaryan
2023-02-23 14:51   ` Hyeonggon Yoo
2023-02-23 14:59     ` Hyeonggon Yoo
2023-02-23 17:46     ` Suren Baghdasaryan [this message]
2023-02-16  5:17 ` [PATCH v3 17/35] mm/mmap: write-lock VMA before shrinking or expanding it Suren Baghdasaryan
2023-02-23 20:20   ` Liam R. Howlett
2023-02-23 20:28     ` Liam R. Howlett
2023-02-23 21:16       ` Suren Baghdasaryan
2023-02-24  1:46         ` Liam R. Howlett
2023-02-24  2:06           ` Suren Baghdasaryan
2023-02-24 16:14             ` Liam R. Howlett
2023-02-24 16:19               ` Suren Baghdasaryan
2023-02-27 17:33                 ` Suren Baghdasaryan
2023-02-16  5:17 ` [PATCH v3 18/35] mm/mremap: write-lock VMA while remapping it to a new address range Suren Baghdasaryan
2023-02-16  5:17 ` [PATCH v3 19/35] mm: write-lock VMAs before removing them from VMA tree Suren Baghdasaryan
2023-02-16  5:17 ` [PATCH v3 20/35] mm: conditionally write-lock VMA in free_pgtables Suren Baghdasaryan
2023-02-16  5:17 ` [PATCH v3 21/35] mm/mmap: write-lock adjacent VMAs if they can grow into unmapped area Suren Baghdasaryan
2023-02-16 15:34   ` Liam R. Howlett
2023-02-16 19:36     ` Suren Baghdasaryan
2023-02-17 14:50       ` Liam R. Howlett
2023-02-17 15:54         ` Suren Baghdasaryan
2023-02-16  5:17 ` [PATCH v3 22/35] kernel/fork: assert no VMA readers during its destruction Suren Baghdasaryan
2023-02-16  5:17 ` [PATCH v3 23/35] mm/mmap: prevent pagefault handler from racing with mmu_notifier registration Suren Baghdasaryan
2023-02-23 20:06   ` Liam R. Howlett
2023-02-23 20:29     ` Suren Baghdasaryan
2023-02-16  5:17 ` [PATCH v3 24/35] mm: introduce vma detached flag Suren Baghdasaryan
2023-02-23 20:08   ` Liam R. Howlett
2023-02-23 20:34     ` Suren Baghdasaryan
2023-02-16  5:17 ` [PATCH v3 25/35] mm: introduce lock_vma_under_rcu to be used from arch-specific code Suren Baghdasaryan
2023-02-16  5:17 ` [PATCH v3 26/35] mm: fall back to mmap_lock if vma->anon_vma is not yet set Suren Baghdasaryan
2023-02-16 15:44   ` Matthew Wilcox
2023-02-16 19:43     ` Suren Baghdasaryan
2023-02-17  2:14       ` Suren Baghdasaryan
2023-02-17 10:21         ` Hyeonggon Yoo
2023-02-17 16:13           ` Suren Baghdasaryan
2023-02-17 18:49             ` Hyeonggon Yoo
2023-02-17 16:05         ` Matthew Wilcox
2023-02-17 16:10           ` Suren Baghdasaryan
2023-04-03 19:49             ` Matthew Wilcox
2023-02-16  5:17 ` [PATCH v3 27/35] mm: add FAULT_FLAG_VMA_LOCK flag Suren Baghdasaryan
2023-02-16  5:17 ` [PATCH v3 28/35] mm: prevent do_swap_page from handling page faults under VMA lock Suren Baghdasaryan
2023-02-16  5:17 ` [PATCH v3 29/35] mm: prevent userfaults to be handled under per-vma lock Suren Baghdasaryan
2023-02-16  5:17 ` [PATCH v3 30/35] mm: introduce per-VMA lock statistics Suren Baghdasaryan
2023-02-16  5:17 ` [PATCH v3 31/35] x86/mm: try VMA lock-based page fault handling first Suren Baghdasaryan
2023-02-16  5:17 ` [PATCH v3 32/35] arm64/mm: " Suren Baghdasaryan
2023-02-16  5:17 ` [PATCH v3 33/35] powerc/mm: " Suren Baghdasaryan
2023-02-16  5:17 ` [PATCH v3 34/35] mm/mmap: free vm_area_struct without call_rcu in exit_mmap Suren Baghdasaryan
2023-02-16  5:17 ` [PATCH v3 35/35] mm: separate vma->lock from vm_area_struct Suren Baghdasaryan
2023-02-24  9:21 ` [PATCH v3 00/35] Per-VMA locks freak07
2023-02-27 16:50   ` Davidlohr Bueso
2023-02-27 17:22     ` Suren Baghdasaryan

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