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Date:   Tue, 23 May 2023 15:34:31 +0200
From:   Jan Kara <jack@suse.cz>
To:     Kent Overstreet <kent.overstreet@linux.dev>
Cc:     Jan Kara <jack@suse.cz>, linux-kernel@vger.kernel.org,
        linux-fsdevel@vger.kernel.org, linux-bcachefs@vger.kernel.org,
        Kent Overstreet <kent.overstreet@gmail.com>,
        "Darrick J . Wong" <djwong@kernel.org>, dhowells@redhat.com,
        Andreas Gruenbacher <agruenba@redhat.com>,
        cluster-devel@redhat.com, Bob Peterson <rpeterso@redhat.com>
Subject: Re: [PATCH 06/32] sched: Add task_struct->faults_disabled_mapping
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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?

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