From mboxrd@z Thu Jan 1 00:00:00 1970 From: Christoph Hellwig Subject: Re: [PATCH 0/2] mm: gup: don't unmap or drop filesystem buffers Date: Mon, 18 Jun 2018 01:10:03 -0700 Message-ID: <20180618081003.GA20927@infradead.org> References: <20180617012510.20139-1-jhubbard@nvidia.com> <010001640fbe0dd8-f999e7f6-7b6e-4deb-b073-0c572006727d-000000@email.amazonses.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <010001640fbe0dd8-f999e7f6-7b6e-4deb-b073-0c572006727d-000000@email.amazonses.com> Sender: linux-kernel-owner@vger.kernel.org To: Christopher Lameter Cc: john.hubbard@gmail.com, Matthew Wilcox , Michal Hocko , Jason Gunthorpe , Dan Williams , Jan Kara , linux-mm@kvack.org, LKML , linux-rdma , John Hubbard List-Id: linux-rdma@vger.kernel.org On Sun, Jun 17, 2018 at 09:54:31PM +0000, Christopher Lameter wrote: > On Sat, 16 Jun 2018, john.hubbard@gmail.com wrote: > > > I've come up with what I claim is a simple, robust fix, but...I'm > > presuming to burn a struct page flag, and limit it to 64-bit arches, in > > order to get there. Given that the problem is old (Jason Gunthorpe noted > > that RDMA has been living with this problem since 2005), I think it's > > worth it. > > > > Leaving the new page flag set "nearly forever" is not great, but on the > > other hand, once the page is actually freed, the flag does get cleared. > > It seems like an acceptable tradeoff, given that we only get one bit > > (and are lucky to even have that). > > This is not robust. Multiple processes may register a page with the RDMA > subsystem. How do you decide when to clear the flag? I think you would > need an additional refcount for the number of times the page was > registered. And it's not just RDMA that is using get_user_pages. We have tons of users that do short, spurious get_user_pages do do zero copy operations. We can't leave the page in a wrecked state after that. > I still think the cleanest solution here is to require mmu notifier > callbacks and to not pin the page in the first place. If a NIC does not > support a hardware mmu then it can still simulate it in software by > holding off the ummapping the mmu notifier callback until any pending > operation is complete and then invalidate the mapping so that future > operations require a remapping (or refaulting). Sounds ok for RDMA, not going to help for most other users.