From mboxrd@z Thu Jan 1 00:00:00 1970 From: Matthew Wilcox Subject: Re: [PATCH 00/34] put_user_pages(): miscellaneous call sites Date: Fri, 2 Aug 2019 07:24:43 -0700 Message-ID: <20190802142443.GB5597@bombadil.infradead.org> References: <20190802022005.5117-1-jhubbard@nvidia.com> <20190802091244.GD6461@dhcp22.suse.cz> <20190802124146.GL25064@quack2.suse.cz> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: <20190802124146.GL25064@quack2.suse.cz> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=m.gmane.org@lists.infradead.org To: Jan Kara Cc: linux-fbdev@vger.kernel.org, kvm@vger.kernel.org, Dave Hansen , Dave Chinner , dri-devel@lists.freedesktop.org, Michal Hocko , linux-mm@kvack.org, sparclinux@vger.kernel.org, Ira Weiny , Dan Williams , devel@driverdev.osuosl.org, rds-devel@oss.oracle.com, linux-rdma@vger.kernel.org, x86@kernel.org, amd-gfx@lists.freedesktop.org, Christoph Hellwig , Jason Gunthorpe , xen-devel@lists.xenproject.org, devel@lists.orangefs.org, linux-media@vger.kernel.org, John Hubbard , intel-gfx@lists.freedesktop.org, john.hubbard@gmail.com, linux-block@vger.kernel.org, =?iso-8859-1?B?Suly9G1l?= Glisse , linux-rpi-kernel@lists.infradead.org, ceph-devel@vger.kernel.org, linux-arm-kernel@lists.infradead.o List-Id: dri-devel@lists.freedesktop.org On Fri, Aug 02, 2019 at 02:41:46PM +0200, Jan Kara wrote: > On Fri 02-08-19 11:12:44, Michal Hocko wrote: > > On Thu 01-08-19 19:19:31, john.hubbard@gmail.com wrote: > > [...] > > > 2) Convert all of the call sites for get_user_pages*(), to > > > invoke put_user_page*(), instead of put_page(). This involves dozens of > > > call sites, and will take some time. > > > > How do we make sure this is the case and it will remain the case in the > > future? There must be some automagic to enforce/check that. It is simply > > not manageable to do it every now and then because then 3) will simply > > be never safe. > > > > Have you considered coccinele or some other scripted way to do the > > transition? I have no idea how to deal with future changes that would > > break the balance though. > > Yeah, that's why I've been suggesting at LSF/MM that we may need to create > a gup wrapper - say vaddr_pin_pages() - and track which sites dropping > references got converted by using this wrapper instead of gup. The > counterpart would then be more logically named as unpin_page() or whatever > instead of put_user_page(). Sure this is not completely foolproof (you can > create new callsite using vaddr_pin_pages() and then just drop refs using > put_page()) but I suppose it would be a high enough barrier for missed > conversions... Thoughts? I think the API we really need is get_user_bvec() / put_user_bvec(), and I know Christoph has been putting some work into that. That avoids doing refcount operations on hundreds of pages if the page in question is a huge page. Once people are switched over to that, they won't be tempted to manually call put_page() on the individual constituent pages of a bvec.