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.
