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Fri, 27 Nov 2020 10:28:54 -0800 (PST) Received: from xz-x1 ([142.126.81.247]) by smtp.gmail.com with ESMTPSA id v28sm6533054qkj.103.2020.11.27.10.28.53 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 27 Nov 2020 10:28:53 -0800 (PST) Date: Fri, 27 Nov 2020 13:28:52 -0500 From: Peter Xu To: David Hildenbrand Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, Andrew Morton , Hugh Dickins , Andrea Arcangeli , Mike Rapoport Subject: Re: [PATCH] mm: Don't fault around userfaultfd-registered regions on reads Message-ID: <20201127182852.GD6573@xz-x1> References: <20201126222359.8120-1-peterx@redhat.com> MIME-Version: 1.0 In-Reply-To: Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=peterx@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline 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, Nov 27, 2020 at 06:00:44PM +0100, David Hildenbrand wrote: > On 26.11.20 23:23, Peter Xu wrote: > > Faulting around for reads are in most cases helpful for the performance so that > > continuous memory accesses may avoid another trip of page fault. However it > > may not always work as expected. > > > > For example, userfaultfd registered regions may not be the best candidate for > > pre-faults around the reads. > > Are we getting uffd faults even though no-one actually accessed it? Userfaultfd should only notify if someone accessed it. > So in case I would track what some part of my program actually reads, I > would get wrong notifications? For shmem, we can't track it right now, afaict. The message will only generate if the page cache is not there. While tracking page reads without page cache is helpless.. because they're all zeros. Thanks, -- Peter Xu