From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Date: Fri, 14 Dec 2018 11:48:43 -0800 From: Matthew Wilcox To: Dan Williams Cc: John Hubbard , david , =?iso-8859-1?B?Suly9G1l?= Glisse , Jan Kara , John Hubbard , Andrew Morton , Linux MM , tom@talpey.com, Al Viro , benve@cisco.com, Christoph Hellwig , Christopher Lameter , "Dalessandro, Dennis" , Doug Ledford , Jason Gunthorpe , Michal Hocko , Mike Marciniszyn , rcampbell@nvidia.com, Linux Kernel Mailing List , linux-fsdevel , Dave Hansen Subject: Re: [PATCH 1/2] mm: introduce put_user_page*(), placeholder versions Message-ID: <20181214194843.GG10600@bombadil.infradead.org> References: <20181208022445.GA7024@redhat.com> <20181210102846.GC29289@quack2.suse.cz> <20181212150319.GA3432@redhat.com> <20181212214641.GB29416@dastard> <20181212215931.GG5037@redhat.com> <20181213005119.GD29416@dastard> <05a68829-6e6d-b766-11b4-99e1ba4bc87b@nvidia.com> <01cf4e0c-b2d6-225a-3ee9-ef0f7e53684d@nvidia.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: owner-linux-mm@kvack.org List-ID: On Fri, Dec 14, 2018 at 11:38:59AM -0800, Dan Williams wrote: > On Thu, Dec 13, 2018 at 10:11 PM John Hubbard wrote: > > I don't have an answer for that, so maybe the page->mapping idea is dead already. > > > > So in that case, there is still one more way to do all of this, which is to > > combine ZONE_DEVICE, HMM, and gup/dma information in a per-page struct, and get > > there via basically page->private, more or less like this: > > If we're going to allocate something new out-of-line then maybe we > should go even further to allow for a page "proxy" object to front a > real struct page. This idea arose from Dave Hansen as I explained to > him the dax-reflink problem, and dovetails with Dave Chinner's > suggestion earlier in this thread for dax-reflink. > > Have get_user_pages() allocate a proxy object that gets passed around > to drivers. Something like a struct page pointer with bit 0 set. This > would add a conditional branch and pointer chase to many page > operations, like page_to_pfn(), I thought something like it would be > unacceptable a few years ago, but then HMM went and added similar > overhead to put_page() and nobody balked. > > This has the additional benefit of catching cases that might be doing > a get_page() on a get_user_pages() result and should instead switch to > a "ref_user_page()" (opposite of put_user_page()) as the API to take > additional references on a get_user_pages() result. > > page->index and page->mapping could be overridden by similar > attributes in the proxy, and allow an N:1 relationship of proxy > instances to actual pages. Filesystems could generate dynamic proxies > as well. > > The auxiliary information (dev_pagemap, hmm_data, etc...) moves to the > proxy and stops polluting the base struct page which remains the > canonical location for dirty-tracking and dma operations. > > The difficulties are reconciling the source of the proxies as both > get_user_pages() and filesystem may want to be the source of the > allocation. In the get_user_pages_fast() path we may not be able to > ask the filesystem for the proxy, at least not without destroying the > performance expectations of get_user_pages_fast(). I think we can do better than a proxy object with bit 0 set. I'd go for allocating something like this: struct dynamic_page { struct page; unsigned long vaddr; unsigned long pfn; ... }; and use a bit in struct page to indicate that this is a dynamic page.