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Date:   Tue, 23 May 2023 09:21:56 -0700
From:   Christoph Hellwig <hch@infradead.org>
To:     Jan Kara <jack@suse.cz>
Cc:     Kent Overstreet <kent.overstreet@linux.dev>,
        cluster-devel@redhat.com, "Darrick J . Wong" <djwong@kernel.org>,
        linux-kernel@vger.kernel.org, dhowells@redhat.com,
        linux-bcachefs@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        Kent Overstreet <kent.overstreet@gmail.com>
Subject: Re: [Cluster-devel] [PATCH 06/32] sched: Add
 task_struct->faults_disabled_mapping
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On Tue, May 23, 2023 at 03:34:31PM +0200, Jan Kara wrote:
> 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?

generic/708 is the btrfs version of this.

But I think all of the file systems that have this deadlock are actually
fundamentally broken because they have a mess up locking hierarchy
where page faults take the same lock that is held over the the direct I/
operation.  And the right thing is to fix this.  I have work in progress
for btrfs, and something similar should apply to gfs2, with the added
complication that it probably means a revision to their network
protocol.

I'm absolutely not in favour to add workarounds for thes kind of locking
problems to the core kernel.  I already feel bad for allowing the
small workaround in iomap for btrfs, as just fixing the locking back
then would have avoid massive ratholing.