From mboxrd@z Thu Jan 1 00:00:00 1970 From: Matthew Wilcox Subject: Re: [PATCH 0/2] mm/fs: put_user_page() proposal Date: Mon, 9 Jul 2018 13:00:49 -0700 Message-ID: <20180709200049.GA5335@bombadil.infradead.org> 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: Nicholas Piggin , john.hubbard@gmail.com, Michal Hocko , Christopher Lameter , Jason Gunthorpe , 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: > > > 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 was thinking about writeback, and neglected the fsync case. For fsync, we could copy the "current" contents of the page to a freshly-allocated page and write _that_ to disc? As long as we redirty the real page after the pin is dropped, I think we're fine.