Re: [PATCH 1/3] NFS: Use WRITE_PLUS for hole punches

[Christoph Hellwig <hch@infradead.org>]

On Mon, Oct 28, 2013 at 11:00:16AM -0400, Anna Schumaker wrote:
> +	struct nfs42_write_plus_args args = {
> +		.wp_fh		= NFS_FH(inode),
> +		.wp_stable	= NFS_FILE_SYNC,
> +		.wp_content	= NFS4_CONTENT_HOLE,
> +		.wp_offset	= offset,
> +		.wp_length	= len,
> +		.wp_allocated	= (mode == 0),
> +	};

After spending some time with draft 21 of the NFSv4.2 spec I don't think
we can use WRITE_PLUS for the prealloc mode of fallocate.  The problem
with the NFS4_CONTENT_HOLE arm of WRITE_PLUS is that it is defined to
zero the whole range, while fallocate is defined as being a no-op for
parts of the range that already contain data.  

In addition we'll also need more sanity checks on the flags argument,
there already is a FALLOC_FL_KEEP_SIZE not supportable by the NFS
semantics (not that it nessecarily should), and more may be added
in the future.  Take a a look at the other fallocate implementations tha
t have a quick check on the top for flags they don't support:

	if (mode & ~(FALLOC_FL_KEEP_SIZE | FALLOC_FL_PUNCH_HOLE))
		return -EOPNOTSUPP;