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[2003:cb:c710:9700:7003:7e4d:43dd:7bfd]) by smtp.gmail.com with ESMTPSA id v10-20020adfe28a000000b003063a1cdaf2sm4390604wri.48.2023.05.26.02.24.18 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 26 May 2023 02:24:19 -0700 (PDT) Message-ID: <950f095f-5182-6f14-cdc3-ce5eb35884ca@redhat.com> Date: Fri, 26 May 2023 11:24:18 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.10.0 Subject: Re: [RFC PATCH v2 1/3] mm: Don't pin ZERO_PAGE in pin_user_pages() To: David Howells , Lorenzo Stoakes Cc: Christoph Hellwig , Jens Axboe , Al Viro , Matthew Wilcox , Jan Kara , Jeff Layton , Jason Gunthorpe , Logan Gunthorpe , Hillf Danton , Christian Brauner , Linus Torvalds , linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, Andrew Morton References: <4f479af6-2865-4bb3-98b9-78bba9d2065f@lucifer.local> <89c7f535-8fc5-4480-845f-de94f335d332@lucifer.local> <20230525223953.225496-1-dhowells@redhat.com> <20230525223953.225496-2-dhowells@redhat.com> <520730.1685090615@warthog.procyon.org.uk> <522654.1685092526@warthog.procyon.org.uk> From: David Hildenbrand Organization: Red Hat In-Reply-To: <522654.1685092526@warthog.procyon.org.uk> X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: AE6B91C0012 X-Stat-Signature: rfarnerqyprusyb8thbosyopuc6g6zhw X-HE-Tag: 1685093063-459756 X-HE-Meta: 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 qGh+FW/T 5dOa8QkVh06PzrTWl5buBo5UEXV3zGejHrESc9v9GTztoq9licY5wX64BdWFcQYWGk1LC69kDCMOsh/u8UmDzsmUn8c16eXXjtEkFiU6R8rCJOSY5cAhoXNNig3p8HSYFaDe+Bn2NXkbXsMmVIW2XJEOSr/ApLq8I7Jn1+jXROkmDfGh+lXw1BnNnaXrzTcXXkfc16lJ5Z4LmKV9CxK9pERoVvOlvX14SNf5jCOkMmssLVGEXw1aBkluecukL9jmh23yl2ZYkkS5Fh1AMDCs7c7dnFFZTEu3BSlysK7oOZ80fA3PhPLkSu1o7n74Je5EMdHOGfsiZWRJm93FF+resYnwih4cdQ/lre5670EQPK1nwFzeUFuAssZ7pusnS7OduvsuOOkWhgCJAE70M8SckDXYc/CT9PljIhZtRc5L8ZLP54GjBV2f9GVv4TkZ2FkcVKpbX59w340k/RVE= 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 26.05.23 11:15, David Howells wrote: > Lorenzo Stoakes wrote: > >>> iov_iter_extract_pages(), on the other hand, is only used in two places >>> with these patches and the pins are always released with >>> unpin_user_page*() so it's a lot easier to audit. >> >> Thanks for the clarification. I guess these are the cases where you're >> likely to see zero page usage, but since this is changing all PUP*() callers >> don't you need to audit all of those too? > > I don't think it should be necessary. This only affects pages obtained from > gup with FOLL_PIN - and, so far as I know, those always have to be released > with unpin_user_page*() which is part of the gup API and thus it should be > transparent to the users. Right, and even code like like 873aefb376bb ("vfio/type1: Unpin zero pages") would handle it transparently, because they also call unpin_user_page(). [we can remove 873aefb376bb even without this change way because it uses FOLL_LONGTERM that shouldn't return the shared zeropage anymore ] -- Thanks, David / dhildenb