From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jason Gunthorpe Subject: Re: [PATCH 0/2] mm/fs: put_user_page() proposal Date: Mon, 9 Jul 2018 13:56:57 -0600 Message-ID: <20180709195657.GA29026@ziepe.ca> References: <20180709080554.21931-1-jhubbard@nvidia.com> <20180709184937.7a70c3aa@roar.ozlabs.ibm.com> <20180709160806.xjt2l2pbmyiutbyi@quack2.suse.cz> <20180709171651.GE2662@bombadil.infradead.org> <20180709194740.rymbt2fzohbdmpye@quack2.suse.cz> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20180709194740.rymbt2fzohbdmpye@quack2.suse.cz> Sender: linux-kernel-owner@vger.kernel.org To: Jan Kara Cc: Matthew Wilcox , Nicholas Piggin , john.hubbard@gmail.com, Michal Hocko , Christopher Lameter , Dan Williams , Al Viro , linux-mm@kvack.org, LKML , linux-rdma , linux-fsdevel@vger.kernel.org, John Hubbard List-Id: linux-rdma@vger.kernel.org On Mon, Jul 09, 2018 at 09:47:40PM +0200, Jan Kara wrote: > On Mon 09-07-18 10:16:51, Matthew Wilcox wrote: > > On Mon, Jul 09, 2018 at 06:08:06PM +0200, Jan Kara wrote: > > > On Mon 09-07-18 18:49:37, Nicholas Piggin wrote: > > > > The problem with blocking in clear_page_dirty_for_io is that the fs is > > > > holding the page lock (or locks) and possibly others too. If you > > > > expect to have a bunch of long term references hanging around on the > > > > page, then there will be hangs and deadlocks everywhere. And if you do > > > > not have such log term references, then page lock (or some similar lock > > > > bit) for the duration of the DMA should be about enough? > > > > > > There are two separate questions: > > > > > > 1) How to identify pages pinned for DMA? We have no bit in struct page to > > > use and we cannot reuse page lock as that immediately creates lock > > > inversions e.g. in direct IO code (which could be fixed but then good luck > > > with auditing all the other GUP users). Matthew had an idea and John > > > implemented it based on removing page from LRU and using that space in > > > struct page. So we at least have a way to identify pages that are pinned > > > and can track their pin count. > > > > > > 2) What to do when some page is pinned but we need to do e.g. > > > clear_page_dirty_for_io(). After some more thinking I agree with you that > > > just blocking waiting for page to unpin will create deadlocks like: > > > > Why are we trying to writeback a page that is pinned? It's presumed to > > be continuously redirtied by its pinner. We can't evict it. > > So what should be a result of fsync(file), where some 'file' pages are > pinned e.g. by running direct IO? If we just skip those pages, we'll lie to > userspace that data was committed while it was not (and it's not only about > data that has landed in those pages via DMA, you can have first 1k of a page > modified by normal IO in parallel to DMA modifying second 1k chunk). If > fsync(2) returns error, it would be really unexpected by userspace and most > apps will just not handle that correctly. So what else can you do than > block? I think as a userspace I would expect the 'current content' to be flushed without waiting.. If you block fsync() then anyone using a RDMA MR with it will just dead lock. What happens if two processes open the same file and one makes a MR and the other calls fsync()? Sounds bad. Jason