Re: [RFC][PATCH 0/5] Fiemap, an extent mapping ioctl

[Andreas Dilger <adilger@sun.com>]

On May 28, 2008  10:04 -0700, Joel Becker wrote:
> 	It's not about net reduction of code.  It's about a readable and
> understandable interface.  "Pass this array of extent structures we'll
> ignore if you set this special flag" is pretty ugly.  Calling FIECOUNT
> separately is nice.  How it is implemented in the kernel is a whole
> 'nother ball of wax - maybe we don't have ->fiecount() and always
> implement FIECOUNT in terms of a ->fiemap() walk.  Doesn't matter.

In the FIEMAP + NUM_EXTENTS count, you don't need to pass the array of
structures, just the header.  I don't think this is an onerous interface:

	struct fiemap fiemap = { .fm_start = 0, .fm_length = ~0ULL,
				 .fm_num_extents = 0,
				 .fm_flags = FIEMAP_FLAG_NUM_EXTENTS }

	rc = ioctl(fd, FIEMAP, &fiemap);

	if (rc == 0)
		num_extents = fiemap.fm_mapped_extents;

We wouldn't need to even specify .fm_num_extents, if the VFS handler
doesn't check its validity for FIEMAP_FLAG_NUM_EXTENTS (contrary to
my recent change proposal).

Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.