linux-kernel - Re: [PATCH v3 16/35] mm/mmap: write-lock VMAs before merging, splitting or expanding them

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Date:   Thu, 23 Feb 2023 09:46:06 -0800
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Hyeonggon Yoo <42.hyeyoo@...il.com>
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Subject: Re: [PATCH v3 16/35] mm/mmap: write-lock VMAs before merging,
 splitting or expanding them

On Thu, Feb 23, 2023 at 6:51 AM Hyeonggon Yoo <42.hyeyoo@...il.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
>
> --
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