From mboxrd@z Thu Jan 1 00:00:00 1970 From: Christopher Lameter Subject: Re: [PATCH v2 5/6] mm: track gup pages with page->dma_pinned_* fields Date: Thu, 5 Jul 2018 14:17:19 +0000 Message-ID: <010001646acdf1d8-0460be04-cc74-4a2d-be89-a337461bd485-000000@email.amazonses.com> References: <20180702005654.20369-1-jhubbard@nvidia.com> <20180702005654.20369-6-jhubbard@nvidia.com> <20180702095331.n5zfz35d3invl5al@quack2.suse.cz> <010001645d77ee2c-de7fedbd-f52d-4b74-9388-e6435973792b-000000@email.amazonses.com> <01000164611dacae-5ac25e48-b845-43ef-9992-fc1047d8e0a0-000000@email.amazonses.com> <3c71556f-1d71-873a-6f74-121865568bf7@nvidia.com> <20180704104318.f5pnqtnn3unkwauw@quack2.suse.cz> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Return-path: In-Reply-To: <20180704104318.f5pnqtnn3unkwauw@quack2.suse.cz> Sender: linux-kernel-owner@vger.kernel.org To: Jan Kara Cc: John Hubbard , john.hubbard@gmail.com, Matthew Wilcox , Michal Hocko , Jason Gunthorpe , Dan Williams , linux-mm@kvack.org, LKML , linux-rdma , linux-fsdevel@vger.kernel.org List-Id: linux-rdma@vger.kernel.org On Wed, 4 Jul 2018, Jan Kara wrote: > > So this seems unsolvable without having the caller specify that it knows the > > page type, and that it is therefore safe to decrement page->dma_pinned_count. > > I was hoping I'd found a way, but clearly I haven't. :) > > Well, I think the misconception is that "pinned" is a fundamental property > of a page. It is not. "pinned" is a property of a page reference (i.e., a > kind of reference that can be used for DMA access) and page gets into > "pinned" state if it has any reference of "pinned" type. And when you > realize this, it is obvious that you just have to have a special api for > getting and dropping references of this "pinned" type. For getting we > already have get_user_pages(), for putting we have to create the api... Maybe we can do something by creating a special "pinned" bit in the pte? If it is a RDMA reference then set that pinned bit there. Thus any of the references could cause a pin. Since the page struct does not contain that information we therefore have to scan through the ptes to figure out if a page is pinned? If so then we would not need a special function for dropping the reference. References to a page can also be created from devices mmu. Maybe we could at some point start to manage them in a similar way to the page tables of the processor? The mmu notifiers are a bit awkward if we need closer mm integration.