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

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Message-ID: <Y/d9b5Ev6vAEzzVO@localhost>
Date:   Thu, 23 Feb 2023 14:51:27 +0000
From:   Hyeonggon Yoo <42.hyeyoo@...il.com>
To:     Suren Baghdasaryan <surenb@...gle.com>
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Subject: Re: [PATCH v3 16/35] mm/mmap: write-lock VMAs before merging,
 splitting or expanding them

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()?

> 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?

--
Regards,
Hyeonggon