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From:   Suren Baghdasaryan <surenb@google.com>
Date:   Tue, 17 Jan 2023 17:53:00 -0800
Message-ID: <CAJuCfpH8-wNiPXQcS=0j-Ex7iMqoBkUhjSN8QiAvq6FdXudRGQ@mail.gmail.com>
Subject: Re: [PATCH 26/41] kernel/fork: assert no VMA readers during its destruction
To:     Michal Hocko <mhocko@suse.com>
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On Tue, Jan 17, 2023 at 7:42 AM 'Michal Hocko' via kernel-team
<kernel-team@android.com> wrote:
>
> On Mon 09-01-23 12:53:21, Suren Baghdasaryan wrote:
> > Assert there are no holders of VMA lock for reading when it is about to be
> > destroyed.
> >
> > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> > ---
> >  include/linux/mm.h | 8 ++++++++
> >  kernel/fork.c      | 2 ++
> >  2 files changed, 10 insertions(+)
> >
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index 594e835bad9c..c464fc8a514c 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -680,6 +680,13 @@ static inline void vma_assert_write_locked(struct vm_area_struct *vma)
> >       VM_BUG_ON_VMA(vma->vm_lock_seq != READ_ONCE(vma->vm_mm->mm_lock_seq), vma);
> >  }
> >
> > +static inline void vma_assert_no_reader(struct vm_area_struct *vma)
> > +{
> > +     VM_BUG_ON_VMA(rwsem_is_locked(&vma->lock) &&
> > +                   vma->vm_lock_seq != READ_ONCE(vma->vm_mm->mm_lock_seq),
> > +                   vma);
>
> Do we really need to check for vm_lock_seq? rwsem_is_locked should tell
> us something is wrong on its own, no? This could be somebody racing with
> the vma destruction and using the write lock. Unlikely but I do not see
> why to narrow debugging scope.

I wanted to ensure there are no page fault handlers (read-lockers)
when we are destroying the VMA and rwsem_is_locked(&vma->lock) alone
could trigger if someone is concurrently calling vma_write_lock(). But
I don't think we expect someone to be write-locking the VMA while we
are destroying it, so you are right, I'm overcomplicating things here.
I think I can get rid of vma_assert_no_reader() and add
VM_BUG_ON_VMA(rwsem_is_locked(&vma->lock)) directly in
__vm_area_free(). WDYT?


> --
> Michal Hocko
> SUSE Labs
>
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