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From:   Suren Baghdasaryan <surenb@google.com>
Date:   Wed, 18 Jan 2023 13:33:00 -0800
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Subject: Re: [PATCH 28/41] mm: introduce lock_vma_under_rcu to be used from
 arch-specific code
To:     Matthew Wilcox <willy@infradead.org>
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On Tue, Jan 17, 2023 at 6:44 PM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Tue, Jan 17, 2023 at 05:06:57PM -0800, Suren Baghdasaryan wrote:
> > On Tue, Jan 17, 2023 at 7:47 AM Michal Hocko <mhocko@suse.com> wrote:
> > >
> > > On Mon 09-01-23 12:53:23, Suren Baghdasaryan wrote:
> > > > Introduce lock_vma_under_rcu function to lookup and lock a VMA during
> > > > page fault handling. When VMA is not found, can't be locked or changes
> > > > after being locked, the function returns NULL. The lookup is performed
> > > > under RCU protection to prevent the found VMA from being destroyed before
> > > > the VMA lock is acquired. VMA lock statistics are updated according to
> > > > the results.
> > > > For now only anonymous VMAs can be searched this way. In other cases the
> > > > function returns NULL.
> > >
> > > Could you describe why only anonymous vmas are handled at this stage and
> > > what (roughly) has to be done to support other vmas? lock_vma_under_rcu
> > > doesn't seem to have any anonymous vma specific requirements AFAICS.
> >
> > TBH I haven't spent too much time looking into file-backed page faults
> > yet but a couple of tasks I can think of are:
> > - Ensure that all vma->vm_ops->fault() handlers do not rely on
> > mmap_lock being read-locked;
>
> I think this way lies madness.  There are just too many device drivers
> that implement ->fault.  My plan is to call the ->map_pages() method
> under RCU without even read-locking the VMA.  If that doesn't satisfy
> the fault, then drop all the way back to taking the mmap_sem for read
> before calling into ->fault.

Sounds reasonable to me but I guess the devil is in the details...

>