On Wed, May 10, 2023 at 03:07:37AM +0200, Jan Kara wrote: > On Tue 09-05-23 12:56:31, Kent Overstreet wrote: > > From: Kent Overstreet <kent.overstreet@xxxxxxxxx> > > > > This is used by bcachefs to fix a page cache coherency issue with > > O_DIRECT writes. > > > > Also relevant: mapping->invalidate_lock, see below. > > > > O_DIRECT writes (and other filesystem operations that modify file data > > while bypassing the page cache) need to shoot down ranges of the page > > cache - and additionally, need locking to prevent those pages from > > pulled back in. > > > > But O_DIRECT writes invoke the page fault handler (via get_user_pages), > > and the page fault handler will need to take that same lock - this is a > > classic recursive deadlock if userspace has mmaped the file they're DIO > > writing to and uses those pages for the buffer to write from, and it's a > > lock ordering deadlock in general. > > > > Thus we need a way to signal from the dio code to the page fault handler > > when we already are holding the pagecache add lock on an address space - > > this patch just adds a member to task_struct for this purpose. For now > > only bcachefs is implementing this locking, though it may be moved out > > of bcachefs and made available to other filesystems in the future. > > It would be nice to have at least a link to the code that's actually using > the field you are adding. Bit of a trick to link to a _later_ patch in the series from a commit message, but... https://evilpiepirate.org/git/bcachefs.git/tree/fs/bcachefs/fs-io.c#n975 https://evilpiepirate.org/git/bcachefs.git/tree/fs/bcachefs/fs-io.c#n2454 > Also I think we were already through this discussion [1] and we ended up > agreeing that your scheme actually solves only the AA deadlock but a > malicious userspace can easily create AB BA deadlock by running direct IO > to file A using mapped file B as a buffer *and* direct IO to file B using > mapped file A as a buffer. No, that's definitely handled (and you can see it in the code I linked), and I wrote a torture test for fstests as well. David Howells was also just running into a strange locking situation with iov_iters and recursive gups - I don't recall all the details, but it sounded like this might be a solution for that. David, did you have thoughts on that?
