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
