linux-kernel - Re: [PATCH 3/3] btrfs: set FS_SUPPORTS_SEEK

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Message-ID: <20150427154133.65f58366@notabene.brown>
Date:	Mon, 27 Apr 2015 15:41:33 +1000
From:	NeilBrown <neilb@...e.de>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	David Howells <dhowells@...hat.com>, Chris Mason <clm@...com>,
	Al Viro <viro@...IV.linux.org.uk>, Josef Bacik <jbacik@...com>,
	David Sterba <dsterba@...e.cz>,
	Dave Chinner <david@...morbit.com>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-btrfs@...r.kernel.org
Subject: Re: [PATCH 3/3] btrfs: set FS_SUPPORTS_SEEK_HOLE flag.

On Mon, 20 Apr 2015 02:48:55 -0700 Christoph Hellwig <hch@...radead.org>
wrote:

> On Mon, Apr 20, 2015 at 10:46:49AM +0100, David Howells wrote:
> > NeilBrown <neilb@...e.de> wrote:
> > 
> > > Missing patch 2 of the 3-patch series?
> > 
> > Yes. :-)
> > 
> > Do ext4 and xfs support this, do you know?
> 
> Yes.  As do f2fs, ocfs2, gfs2, ceph and NFSv4.2

Are you sure about NFSv4.2?

I see that it *can* report holes, but is there any guarantee that if you
create a new file and write only the 5th block, then READ_PLUS will reliably
report that the first 4 block are holes??

Because if it doesn't guarantee that, then NFSv4.2 doesn't fit the with the
others where SEEK_HOLE reliable reports holes.
On the other hand if NFSv4.2 *does* guarantee that then the current READ_PLUS
server patches are broken because they just use vfs_llseek and assume that
trust what it says.

It would be really nice if SEEK_{DATA,HOLE} either reported holes reliably or
returned ENXIO, but I guess there was a goo reason not to do that.

Thanks,
NeilBrown

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