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

[Mark Fasheh <mfasheh@suse.com>]

On Sun, May 25, 2008 at 03:42:03PM -0400, Christoph Hellwig wrote:

[ I'll get back to you regarding luns and the HSM flag ]

> > struct fiemap_extent_info {
> > 	unsigned int	fi_flags;		/* Flags as passed from user */
> > 	unsigned int	fi_extents_mapped;	/* Number of mapped extents */
> > 	unsigned int	fi_extents_max;		/* Size of fiemap_extent array */
> > 	char		*fi_extents_start;	/* Start of fiemap_extent array */
> > };
> 
> Why is this passes a structure instead of individual arguments?

[ The structure vs args seems to been addressed elsewhere in
this thread ]

> Also why isn't fi_extents_start properly typed?

There's no good reason, my brain just wrote it that way. I can type it
properly in my next patch.


> > If the request has the FIEMAP_FLAG_NUM_EXTENTS flag set, then calling
> > this helper is not necessary and fi_extents_mapped can be set
> > directly.
> 
> Sounds like the count number of extents request should be a separate
> ioctl and separate filesystem entry point instead of overloading FIEMAP.
> 
> Just define a simple FIECOUNT ioctl.

Having it as a seperate, simpler ioctl is fine with me. As it is, we're sort
of shoe-horning it into a structure which is more optimized for returning
actual extent data. The flag breaks lots of rules for that structure, such
as no actual fm_extents needing to be allocated, fm_extent_count is
magically ignored for that call, etc. So the simpler API to userspace is a
win, IMHO.

What about the back-end though? This is pretty transparently handled in
fiemap_fill_next_extent() and many file systems (Ocfs2 included) would just
have ->fiecount callbacks that are nearly identical ->fiecount to their
->fiemap...
	--Mark

--
Mark Fasheh