Re: [PATCH 06/32] sched: Add task_struct->faults_disabled_mapping

Andreas Grünbacher
2023-05-25T22:05:06Z

Am Di., 23. Mai 2023 um 15:37 Uhr schrieb Jan Kara <jack@suse.cz>:
> On Wed 10-05-23 02:18:45, Kent Overstreet wrote:
> > 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@gmail.com>
> > > >
> > > > 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
>
> Thanks and I'm sorry for the delay.
>
> > > 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.
>
> I've checked the code and AFAICT it is all indeed handled. BTW, I've now
> remembered that GFS2 has dealt with the same deadlocks - b01b2d72da25
> ("gfs2: Fix mmap + page fault deadlocks for direct I/O") - in a different
> way (by prefaulting pages from the iter before grabbing the problematic
> lock and then disabling page faults for the iomap_dio_rw() call). I guess
> we should somehow unify these schemes so that we don't have two mechanisms
> for avoiding exactly the same deadlock. Adding GFS2 guys to CC.
>
> Also good that you've written a fstest for this, that is definitely a useful
> addition, although I suspect GFS2 guys added a test for this not so long
> ago when testing their stuff. Maybe they have a pointer handy?

Ah yes, that's xfstests commit d3cbdabf ("generic: Test page faults
during read and write").

Thanks,
Andreas

> Honza
> --
> Jan Kara <jack@suse.com>
> SUSE Labs, CR