Re: [PATCH 17/41] mm/mmap: move VMA locking before anon_vma_lock_write call

[Suren Baghdasaryan]

On Wed, Jan 18, 2023 at 1:23 AM Michal Hocko <mhocko@xxxxxxxx> wrote:
>
> On Tue 17-01-23 18:01:01, Suren Baghdasaryan wrote:
> > On Tue, Jan 17, 2023 at 7:16 AM Michal Hocko <mhocko@xxxxxxxx> 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.

>
> Withtout that additional reasonaning it is not really clear why that is
> needed and seems arbitrary.

Would the above be a good reasoning?

>
> --
> Michal Hocko
> SUSE Labs