linux-kernel - Re: [PATCH 17/41] mm/mmap: move VMA locking before anon_vma_lock

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Message-ID: <CAJuCfpF3j5Sx+D5p5QPCHP4jcWZUiYm=FfUNYhc6QyHZQvDgpw@mail.gmail.com>
Date:   Wed, 18 Jan 2023 13:48:13 -0800
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     akpm@...ux-foundation.org, michel@...pinasse.org,
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Subject: Re: [PATCH 17/41] mm/mmap: move VMA locking before
 anon_vma_lock_write call

On Wed, Jan 18, 2023 at 1:33 PM Michal Hocko <mhocko@...e.com> wrote:
>
> On Wed 18-01-23 10:09:29, Suren Baghdasaryan wrote:
> > On Wed, Jan 18, 2023 at 1:23 AM Michal Hocko <mhocko@...e.com> wrote:
> > >
> > > On Tue 17-01-23 18:01:01, Suren Baghdasaryan wrote:
> > > > On Tue, Jan 17, 2023 at 7:16 AM Michal Hocko <mhocko@...e.com> wrote:
> > > > >
> > > > > On Mon 09-01-23 12:53:12, Suren Baghdasaryan wrote:
> > > > > > Move VMA flag modification (which now implies VMA locking) before
> > > > > > anon_vma_lock_write to match the locking order of page fault handler.
> > > > >
> > > > > Does this changelog assumes per vma locking in the #PF?
> > > >
> > > > Hmm, you are right. Page fault handlers do not use per-vma locks yet
> > > > but the changelog already talks about that. Maybe I should change it
> > > > to simply:
> > > > ```
> > > > Move VMA flag modification (which now implies VMA locking) before
> > > > vma_adjust_trans_huge() to ensure the modifications are done after VMA
> > > > has been locked.
> > >
> > > Because ....
> >
> > because vma_adjust_trans_huge() modifies the VMA and such
> > modifications should be done under VMA write-lock protection.
>
> So it will become:
> Move VMA flag modification (which now implies VMA locking) before
> vma_adjust_trans_huge() to ensure the modifications are done after VMA
> has been locked. Because vma_adjust_trans_huge() modifies the VMA and such
> modifications should be done under VMA write-lock protection.
>
> which is effectivelly saying
> vma_adjust_trans_huge() modifies the VMA and such modifications should
> be done under VMA write-lock protection so move VMA flag modifications
> before so all of them are covered by the same write protection.
>
> right?

Yes, and the wording in the latter version is simpler to understand
IMO, so I would like to adopt it. Do you agree?

> --
> Michal Hocko
> SUSE Labs