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Received: from viro by zeniv.linux.org.uk with local (Exim 4.96 #2 (Red Hat Linux)) id 1pLCys-004Liy-3A; Fri, 27 Jan 2023 00:52:39 +0000 Date: Fri, 27 Jan 2023 00:52:38 +0000 From: Al Viro To: David Howells Cc: David Hildenbrand , 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 Subject: Re: [PATCH v11 2/8] iov_iter: Add a function to extract a page list from an iterator Message-ID: References: <20230126141626.2809643-1-dhowells@redhat.com> <20230126141626.2809643-3-dhowells@redhat.com> <2907150.1674777410@warthog.procyon.org.uk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <2907150.1674777410@warthog.procyon.org.uk> X-Rspamd-Queue-Id: 14E184000C X-Stat-Signature: kkxj49ogs93wtqf5ez7fdb9b8gqm7bfe X-Rspam-User: X-Rspamd-Server: rspam08 X-HE-Tag: 1674780776-921932 X-HE-Meta: U2FsdGVkX19a+QB3mCQ4VLlrs7JmoXu+GzK9EsjWFDbcwBLe39BHsoTG5WIB6BgcCU/KB18vVseChwcXzzsz/xAMfSKilZnX3IN6O9OSU/e9xEDrqf7k2StNTQcDfUCESiUZ039L+UFZ1m21BFmvyEyh2p2q3dsJu9CvpQyRCmJsyCbagf5E/5nuRBZoCpjmgb0nKM/EzVbHPoKnLMJgZwNKVvcfN+o4rRulHJ6GTfb5s415oHoLvimoSTMch4H1DZmPLmxsByoDdkVCoAfo9U3dSUK1dp0U+1oIQkCREVGOlr2beJ/XRluzQZJa/sQZLzGuucW9/kuWi+xDTb5FhgYVmgYHvi1z6V0IJnrxWpEQGRetwcTieipG4JSMiJ3ATHOgayDiWlJ1C2FLmBh1xwbg5I2iJyeo1mewwekGj3BlU+Dnsm/jFaUpD6W74DVK7HSy5ejXNPs+imMwHvzFnN7DJy3to/15kZjbFmhxXqllKZGB1As+qtX8VN9lrzCIVAVm3sQqBwLag3WogJUJ0NNlrWOHQidSdVkbbtt0DySniyiOBG0/iL6sHWSUEsn33y5dupmBsMgZw6yyIYksVvPj0pKvyJ/bym5VdfntvMdkXkkntqJeh74Xgr6sTGK0LF+N7UsJd8HBmIvzVrpYwxJw4q3yZjzGoJoohgY3lDwSM28Aeembhyfh+zbpJsXvEH9uxiZEQ2Zmu1a/O4MJm6eGgZEcewjgQ6Dl2luPJfl5vTVSkW+4Md6OgFwLXyLHpOkTl3xarZ6skY656nwKRx45+GN/+Mu0prCdGYnjMinJ4c3I/fBDwLnYeErs+Y4fnIDBta90wZ4SX1kyI94VqOiXfRBKrlbcCw2FP1FQDwLVgvcEPmDbRYHn0fwHXUjdkfaytvB317AYZZ9uPuT0T9ob2RkaPkLXlQlz3z3dLdOiuD/0mVLa5UWLOy95zuOvY4ytzwnLIKr8AEs3OEP FS+9ZyAf XDpilM1wmBWL+1QWzXVaqXbHi1Jt0XHa8sTFZg2KcG2ZhFK8EUsFhmKoYAsl2EvrTwtQ35vXX/FVI95Uz4wb+57Ax1TasysiblysIE1b6ZAKo+pGQMNXxUEev+O9lCOcJdTycnpl94OaYLtNREz9uiRrlawyry9qYYHQmmENK05Wq+RBe9Qtc4NlTYGMq4a7+ELzZelsrNzG5jyydnslE9k5s/gkDIyVvinUygM1PPmHoEYDoYsqRCK521YkJj8fX9Tnn2SDtmkhMKyqGwrCvnxbH4doRIIHwdq6QYBO3eCUYT2NzRf0Yna2rXaGmaMYo1aPqKRfc0aQzsA5CkDtU9Di+bkM6srkYnDj66IDKjt3abmXlY8ZmddXSHooWH0Cj7K9X5ATTWYbR+0t5c00bseVtZWNA981JeSfG35zX9XoYYT6tDRdC5xzFOQWM7NNenDe67xOU0DEHascqbOjgkZoORaHt27T8mXE1 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, Jan 26, 2023 at 11:56:50PM +0000, 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. It doesn't; sorry, really confused memories of what's going on, took a while to sort them out (FWIW, writeback is where we unmap and check if page is pinned, while pin_user_pages running into an unmapped page will end up with handle_mm_fault() (->fault(), actually) try to get the sucker locked and block on that until the writeback is over). Said that, I still think that pinned pages (arbitrary pagecache ones, at that) ending up in a pipe is a seriously bad idea. It's trivial to arrange for them to stay that way indefinitely - no priveleges needed, very few limits, etc.