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From:   Souptick Joarder <jrdr.linux@gmail.com>
Date:   Fri, 13 Mar 2020 09:52:32 +0530
Message-ID: <CAFqt6zbh8pQ+UpPLLhQjpr+e=j-RUET7uX=XkG7AQqH9qysQaQ@mail.gmail.com>
Subject: Re: [PATCH] mm/hmm.c : Remove additional check for lockdep_assert_held()
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     =?UTF-8?B?SsOpcsO0bWUgR2xpc3Nl?= <jglisse@redhat.com>,
        Linux-MM <linux-mm@kvack.org>, linux-kernel@vger.kernel.org
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On Fri, Mar 13, 2020 at 9:27 AM Andrew Morton <akpm@linux-foundation.org> wrote:
>
> On Fri, 13 Mar 2020 09:17:22 +0530 Souptick Joarder <jrdr.linux@gmail.com> wrote:
>
> > On Fri, Mar 13, 2020 at 8:28 AM Andrew Morton <akpm@linux-foundation.org> wrote:
> > >
> > > On Fri, 13 Mar 2020 07:41:00 +0530 Souptick Joarder <jrdr.linux@gmail.com> wrote:
> > >
> > > > walk_page_range() already has a check for lockdep_assert_held().
> > > > So additional check for lockdep_assert_held() can be removed from
> > > > hmm_range_fault().
> > > >
> > > > ...
> > > >
> > > > --- a/mm/hmm.c
> > > > +++ b/mm/hmm.c
> > > > @@ -681,7 +681,6 @@ long hmm_range_fault(struct hmm_range *range, unsigned int flags)
> > > >       struct mm_struct *mm = range->notifier->mm;
> > > >       int ret;
> > > >
> > > > -     lockdep_assert_held(&mm->mmap_sem);
> > > >
> > > >       do {
> > > >               /* If range is no longer valid force retry. */
> > >
> > > It isn't very obvious that hmm_range_fault() is and will only be called
> > > from walk_page_range() (is it?)
> > >
> >
> > Sorry Andrew, didn't get this part ?
> > * hmm_range_fault() is and will only be called
> >  from walk_page_range() (is it?) *
>
> The patch assumes that hmm_range_fault() will only ever be called via
> walk_page_range().  How do we know this is the case?  And that it
> always will be the case?
>

Ahh, Sorry, I think change log creates confusion.

The patch assumes that walk_page_range() is called from hmm_range_fault().
currently there are two caller for hmm_range_fault().
drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c, line 859
drivers/gpu/drm/nouveau/nouveau_svm.c, line 544

in both case, * &mm->mmap_sem * lock is taken before calling hmm_range_fault().
Now inside hmm_range_fault() there is a check for
lockdep_assert_held(&mm->mmap_sem)
and again inside loop walk_page_range() is called which cross check
same lockdep_assert_held().

So the idea is to remove the first check
lockdep_assert_held(&mm->mmap_sem) in hmm_range_fault().