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[156.34.49.251]) by smtp.gmail.com with ESMTPSA id 102sm864606uar.11.2019.05.24.06.40.36 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Fri, 24 May 2019 06:40:36 -0700 (PDT) Received: from jgg by mlx.ziepe.ca with local (Exim 4.90_1) (envelope-from ) id 1hUAR1-0003QD-Jx; Fri, 24 May 2019 10:40:35 -0300 Date: Fri, 24 May 2019 10:40:35 -0300 From: Jason Gunthorpe To: linux-rdma@vger.kernel.org, linux-mm@kvack.org, Jerome Glisse , Ralph Campbell , John Hubbard Subject: Re: [RFC PATCH 05/11] mm/hmm: Improve locking around hmm->dead Message-ID: <20190524134035.GA12653@ziepe.ca> References: <20190523153436.19102-1-jgg@ziepe.ca> <20190523153436.19102-6-jgg@ziepe.ca> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190523153436.19102-6-jgg@ziepe.ca> User-Agent: Mutt/1.9.4 (2018-02-28) 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 Thu, May 23, 2019 at 12:34:30PM -0300, Jason Gunthorpe wrote: > From: Jason Gunthorpe > > This value is being read without any locking, so it is just an unreliable > hint, however in many cases we need to have certainty that code is not > racing with mmput()/hmm_release(). > > For the two functions doing find_vma(), document that the caller is > expected to hold mmap_sem and thus also have a mmget(). > > For hmm_range_register acquire a mmget internally as it must not race with > hmm_release() when it sets valid. > > Signed-off-by: Jason Gunthorpe > mm/hmm.c | 27 +++++++++++++++++++-------- > 1 file changed, 19 insertions(+), 8 deletions(-) > > diff --git a/mm/hmm.c b/mm/hmm.c > index ec54be54d81135..d97ec293336ea5 100644 > +++ b/mm/hmm.c > @@ -909,8 +909,10 @@ int hmm_range_register(struct hmm_range *range, > range->start = start; > range->end = end; > > - /* Check if hmm_mm_destroy() was call. */ > - if (mirror->hmm->mm == NULL || mirror->hmm->dead) > + /* > + * We cannot set range->value to true if hmm_release has already run. > + */ > + if (!mmget_not_zero(mirror->hmm->mm)) > return -EFAULT; > > range->hmm = mirror->hmm; > @@ -928,6 +930,7 @@ int hmm_range_register(struct hmm_range *range, > if (!range->hmm->notifiers) > range->valid = true; > mutex_unlock(&range->hmm->lock); > + mmput(mirror->hmm->mm); Hi Jerome, when you revised this patch to move the mmput to hmm_range_unregister() it means hmm_release() cannot run while a range exists, and thus we can have this futher simplification rolled into this patch. Can you update your git? Thanks: diff --git a/mm/hmm.c b/mm/hmm.c index 2a08b78550b90d..ddd05f2ebe739a 100644 --- a/mm/hmm.c +++ b/mm/hmm.c @@ -128,17 +128,17 @@ static void hmm_release(struct mmu_notifier *mn, struct mm_struct *mm) { struct hmm *hmm = container_of(mn, struct hmm, mmu_notifier); struct hmm_mirror *mirror; - struct hmm_range *range; /* hmm is in progress to free */ if (!kref_get_unless_zero(&hmm->kref)) return; - /* Wake-up everyone waiting on any range. */ mutex_lock(&hmm->lock); - list_for_each_entry(range, &hmm->ranges, list) - range->valid = false; - wake_up_all(&hmm->wq); + /* + * Since hmm_range_register() holds the mmget() lock hmm_release() is + * prevented as long as a range exists. + */ + WARN_ON(!list_empty(&hmm->ranges)); mutex_unlock(&hmm->lock); down_write(&hmm->mirrors_sem); @@ -908,9 +908,7 @@ int hmm_range_register(struct hmm_range *range, range->hmm = mm->hmm; kref_get(&range->hmm->kref); - /* - * We cannot set range->value to true if hmm_release has already run. - */ + /* Prevent hmm_release() from running while the range is valid */ if (!mmget_not_zero(mm)) return -EFAULT;