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Date:   Tue, 23 May 2023 23:43:32 -0700
From:   Christoph Hellwig <hch@infradead.org>
To:     Kent Overstreet <kent.overstreet@linux.dev>
Cc:     Christoph Hellwig <hch@infradead.org>, Jan Kara <jack@suse.cz>,
        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 12:35:35PM -0400, Kent Overstreet wrote:
> No, this is fundamentally because userspace controls the ordering of
> locking because the buffer passed to dio can point into any address
> space. You can't solve this by changing the locking heirarchy.
> 
> If you want to be able to have locking around adding things to the
> pagecache so that things that bypass the pagecache can prevent
> inconsistencies (and we do, the big one is fcollapse), and if you want
> dio to be able to use that same locking (because otherwise dio will also
> cause page cache inconsistency), this is the way to do it.

Well, it seems like you are talking about something else than the
existing cases in gfs2 and btrfs, that is you want full consistency
between direct I/O and buffered I/O.  That's something nothing in the
kernel has ever provided, so I'd be curious why you think you need it
and want different semantics from everyone else?