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[91.12.99.135]) by smtp.gmail.com with ESMTPSA id a18-20020a05600c069200b003db305bece4sm2638720wmn.45.2023.01.26.16.10.39 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 26 Jan 2023 16:10:40 -0800 (PST) Message-ID: <3e5df0cf-d2bd-7795-617d-06a3a32fc18b@redhat.com> Date: Fri, 27 Jan 2023 01:10:39 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.6.0 To: David Howells Cc: Al Viro , Christoph Hellwig , Matthew Wilcox , Jens Axboe , Jan Kara , Jeff Layton , Jason Gunthorpe , Logan Gunthorpe , linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, Christoph Hellwig , John Hubbard References: <20230126141626.2809643-1-dhowells@redhat.com> <20230126141626.2809643-3-dhowells@redhat.com> <2907150.1674777410@warthog.procyon.org.uk> From: David Hildenbrand Organization: Red Hat Subject: Re: [PATCH v11 2/8] iov_iter: Add a function to extract a page list from an iterator In-Reply-To: <2907150.1674777410@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-Queue-Id: E775640010 X-Rspamd-Server: rspam01 X-Stat-Signature: trmtb4o6z8r9fpn5sofwynxjh16w7qgz X-HE-Tag: 1674778251-582372 X-HE-Meta: U2FsdGVkX1/DuUDbLNjkcl3RUzZDwqtk5kg9b42b55iMfM+fuWHxLz+yduRrOUsasHqG/uuKP/dUUMTbw5ld5jRvh/mf7oS0uZGB4AX0IumpW2pTCwHlJMAmoDol8xiqYOKlnwpmTLsL/PAs7i5FWC8lF/ldlxvcrThgpiNirpkQQ+lnEfbOjApKcrLCsujwdz7wpKrmkhDx2Kf+6Wc0gbIQI69TTgZ5VQKwulDHVsX8bpWOT3XC9rMhj4/0e1bJVZwcgEWdjh7COvezgvMJ+qqKUyFiC8z9udyhGM3I3wilkz3ET1cEeJvmEmAPy087Os6grveAkz/mvLqITkblunSEy4fvFXNqLj1m5GmOKWksm40mcE5GiSIHTN5+JD2oa9mOswzlkjLMT1htwgHiI9GwL1AQZSFb0NVjpBpitd0sTyXcLqYvC/yAAxg9AywwGF6NMDDfU3uxdYWtm+jPJ57is5SjkKfHakZgUq/60ytI8f8q3AnEPYX5YDBNsGQLwdnBY9x2oqmRwLkJhYGz52+XuK/BKlHvzjEKNp2p1QljpTSbe2x6ThT4eiLXJSQFrf+Qtyti9Z5AWclg51WEw6rAtFueV4h5x7gN4UImYJX3aLdXLSsdYFQ4V6dy+AOvINCgLkHXOJLk91lqqt5qC7ZnctZHuuT1YCo8YxvkI8bMcMkqd9iUgPAortG7Ccw8kX5zGuTqdYLxFwTetbyGK2erCvC/VBQUOccpW+znTuUskYnnSMLnbboAJd7GU7s2EqMBEplY4DqcNd0+OsGpJDM6bDrm2Leq95v5kIpUpHmcUi1nbqDzKVgiRgIW+LBCs82I+ZbCIoztQK4V3Kn5zLFM/iS8C+zErlaDujoacVWr15/3/m9cNnQlBqOay/W9BpVF/onnuc3WhiXLuxvH2SuW0adNkkZlUYFslqQaHqtADhDX6+Y8SUF13kGTq5Ij9FsAmnzTEyxM/w4qeJd GO2Sv6T/ eZFvPbAoC1Rcg9azsCRMEeoOiLIDgDeT5xqiZyzWOTHuemp/nBBuZZKpLXuWTo1N3Qqk+XlFhpH7HjZQAktAWT1efygfVAgof0SxJZzJ0Llry7JTBuRMgpsC6xs7I5FLfK02p9cSK7jJN08M4cNwnxCQwwoOlItgphhndY4tG/Kz79JStNIftmuEVxBCCjsjQbV8XNvWpHUvA7KiOirD3F/sv+DHKAhpyi29gXLTQcNiWb4E/IdTyWbERhMyqgOhDiS3kTSncT/RwOMnI6XqwhbIxmFy0we4Y1q4fQ0m1xgx3uR5kccBntneuBrvQeE3zgkbBX2T3v/8iVM4jm7awySRYcRZiE+RdfNvk3Uh9PttoFjBNOmS1rtlcgDDSJmJfJV9WQEMrDkeHD3/NFziSBALi2HCXNi+DXdF9ClN5qhBVDGkrH3cxSIN8e765c2M+nvp+VbuXaycy9vrbDnTTCAfGjdkbDbl7M9OkIwfLkxGomkHIsT5t0BBxbhbaFEcVrq9kuYnfPB18DocG8ySu3kxLRQ== 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 27.01.23 00:56, David Howells wrote: > Al says that pinning a page (ie. FOLL_PIN) could cause a deadlock if a page is > vmspliced into a pipe with the pipe holding a pin on it because pinned pages > are removed from all page tables. Is this actually the case? I can't see > offhand where in mm/gup.c it does this. Pinning a page is mostly taking a "special" reference on the page, indicating to the system that the page maybe pinned. For an ordinary order-0 page, this is increasing the refcount by 1024 instead of 1. In addition, we'll do some COW-unsharing magic depending on the page type (e.g., anon vs. fike-backed), and FOLL_LONGTERM. So if the page is mapped R/O only and we want to pin it R/O (!FOLL_WRITE), we might replace it in the page table by a different page via a fault (FAULT_FLAG_UNSHARE). Last but not least, with FOLL_LONGTERM we will make sure to migrate the target page off of MIGRATE_MOVABLE/CMA memory where the unmovable page (while pinned) could otherwise cause trouble (e.g., blocking memory hotunplug). So again, we'd replace it in the page tale by a different page via a fault. In all cases, the page won't be unmapped from the page table. -- Thanks, David / dhildenb