From mboxrd@z Thu Jan  1 00:00:00 1970
From: Christoph Hellwig <hch@infradead.org>
Subject: Re: [PATCH/RFC] fscache/cachefiles versus btrfs
Date: Mon, 13 Apr 2015 10:30:40 -0700
Message-ID: <20150413173040.GA30623@infradead.org>
References: <20150409235234.GJ13731@dastard>
 <20150409174916.5a2efef5@notabene.brown>
 <29536.1428571388@warthog.procyon.org.uk>
 <9282.1428672496@warthog.procyon.org.uk>
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Cc: Dave Chinner <david@fromorbit.com>, NeilBrown <neilb@suse.de>,
	linux-btrfs@vger.kernel.org, linux-cachefs@redhat.com,
	linux-fsdevel@vger.kernel.org, linux-nfs@vger.kernel.org
To: David Howells <dhowells@redhat.com>
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On Fri, Apr 10, 2015 at 02:28:16PM +0100, David Howells wrote:
> Dave Chinner <david@fromorbit.com> wrote:
> 
> > SEEK_HOLE/SEEK_DATA is what you want, as they are page cache
> > coherent, not extent based operations. And, really if you need it to
> > really be able to find real holes, then a superblock flag might be a
> > better way of marking filesystems with the required capability.
> 
> Actually, I wonder if what I want is a kernel_read() that returns ENODATA upon
> encountering a hole at the beginning of the area to be read.

NFS READ_PLUS could also make use of this, but someone needs to actually
implement it.

Until we have that lseek SEEK_HOLE/DATA is the way to go, and the
horrible ->bmap hack needs to die ASAP, I can't believe you managed to
sneak that in in the not too distant past.