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Registered in England and Wales under Company Registration No. 3798903 From: David Howells In-Reply-To: <4f479af6-2865-4bb3-98b9-78bba9d2065f@lucifer.local> 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> To: Lorenzo Stoakes Cc: dhowells@redhat.com, Christoph Hellwig , David Hildenbrand , 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 Subject: Re: [RFC PATCH v2 1/3] mm: Don't pin ZERO_PAGE in pin_user_pages() MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-ID: <522653.1685092526.1@warthog.procyon.org.uk> Date: Fri, 26 May 2023 10:15:26 +0100 Message-ID: <522654.1685092526@warthog.procyon.org.uk> X-Scanned-By: MIMEDefang 3.1 on 10.11.54.9 X-Rspamd-Queue-Id: 323D540011 X-Rspam-User: X-Rspamd-Server: rspam05 X-Stat-Signature: k7gikfbkmopij47jii9osq414he91kop X-HE-Tag: 1685092536-339008 X-HE-Meta: U2FsdGVkX1/ZHq+x177QeZ+iJKFL3P83Sv7M/FHMO3IHA1EUR4R/aEpUe9iaHP0nepxVTPlXTBCs8l78YWf8FaGghol/L7uo8fMiF9y1DIgnOb8hoZMLIgpwD++6VxR7L2YNZ//KYm9aXjS7Zfb9JJZGKUFj8/qIurNgprZW1tcVriarvUHXDMue7ZGGGlJt8Z1IqULTint+gkOjdGxosUb8Y5yQB1FdY4cHb+KuR4C8FWv8WSG02TpoFZBLFJHsoIZWoCmER563YuGElM9CQSOcJB9o5tsZ5VppPWxmi0phoizCzXKuLmy4brlob6wofhGgD+jjiPcvefRWPr+iAi7lvomTmCP8+olUAIznkAmKxL0bMbWSdwIY8FjOgU6YP92INKLwt5NC41e9e8LvY9y+BczzXQ1w5lAJkyATIOIVprWfoH34fBfzFEA8i4l74uVJRDjJw8DX2+6jOh65R3wj542OGxTH1HGfU7vexpix2yjV7u20OV/oS5aZSmp8qcBXjcwoee5ERrm9TN1mOTWANSDT+Xz85X6qm9fuL4PdUcQFPTs7y3cTQN7yCsnFiJJr+8HXa+NomgpNHRDaJdOO7hlPuCwUQa0whJszf9YChyTexi213DQiNeR1b3dir+0+j34kENF9DYlTs7dpYymvmQazPdZthEogk8Or+2dK3oita4kOCGYO1t5ICYCOhH7FdHaTB9kkNCqxWFUMV+4UpYkQFOW8e35V2BID8IroTkwOvV1fXUFXhhFpW25ijBmBsO6kbtnUFx/e2qYsfuLQ0Oeh2qIbjCgvdyRBRan5PTLJDMdEF+1khQ3jtR6JLzRduoUM+5xHZXfOR2ZXT9/2pLKVFvGyvBnRIl7EauuI/vFYDTybjiK+FpeMW62s1fvM7ZAFbSDCVcsnnF9XdDt6IocC1QySTCpL+T8hYG3mKDjYTmgdf34tgvL8EID3dnMkF+QkFFhguto4uIr dAs22GZP Lakj9fpqRq4NovJeifvAs5Tw9/xT5HkOUv1//YV2Jw4vlAfU7JBM6x2KaKzs0pGR7acNLhqSQJt+hs/yj+aSqMPZjs+ivnqTsVHSp9CplMpGGh8KDFePhD5fCj3CPhuHJ0XtJG0ydJWFG0wEEHhLAfWko8xnpEwoqauvnwiR6aqgfwj+3Z27MciKgvLUXlo+7IKCaNVpUUnL8ZaQ9bQk6P/8MXr3invnWc71S 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: 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. Pages obtained FOLL_GET, on the other hand, aren't freed through the gup API - and there are a bunch of ways of releasing them - and getting additional refs too. David