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. > ext4_writepages() ext4_direct_IO_write() > __blockdev_direct_IO() > iov_iter_get_pages() > - pins page > handle = ext4_journal_start_with_reserve(inode, ...) > - starts transaction > ... > lock_page(page) > mpage_submit_page() > clear_page_dirty_for_io(page) -> blocks on pin I don't think it should block. It should fail.
