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/d/Vs1R/+ldNeuG@localhost>
Date:   Thu, 23 Feb 2023 14:59:34 +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 Thu, Feb 23, 2023 at 02:51:27PM +0000, Hyeonggon Yoo 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()?
> 
> > 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.

*Not interval tree, as it can be modified by other processes*

> 
> Or am I missing something?
> 
> --
> Regards,
> Hyeonggon
>