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Thu, 12 Mar 2020 21:22:44 -0700 (PDT) MIME-Version: 1.0 References: <1584065460-22205-1-git-send-email-jrdr.linux@gmail.com> <20200312195850.29693d4e55ec27ae11443c0f@linux-foundation.org> <20200312205741.e97a201037103bbf51e1df40@linux-foundation.org> In-Reply-To: <20200312205741.e97a201037103bbf51e1df40@linux-foundation.org> From: Souptick Joarder Date: Fri, 13 Mar 2020 09:52:32 +0530 Message-ID: Subject: Re: [PATCH] mm/hmm.c : Remove additional check for lockdep_assert_held() To: Andrew Morton Cc: =?UTF-8?B?SsOpcsO0bWUgR2xpc3Nl?= , Linux-MM , linux-kernel@vger.kernel.org Content-Type: text/plain; charset="UTF-8" X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Fri, Mar 13, 2020 at 9:27 AM Andrew Morton wrote: > > On Fri, 13 Mar 2020 09:17:22 +0530 Souptick Joarder wrote: > > > On Fri, Mar 13, 2020 at 8:28 AM Andrew Morton wrote: > > > > > > On Fri, 13 Mar 2020 07:41:00 +0530 Souptick Joarder 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 /* If range is no longer valid force retry.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().