Re: Ext4 fiemap implementation

["Theodore Y. Ts'o" <tytso@mit.edu>]

On Fri, Jun 01, 2018 at 10:01:54AM -0500, Eric Sandeen wrote:
> > Ted, is there any restriction why ext4_fiemap isn't using iomap_fiemap()? Or any
> > reason why ext4 fiemap always returns the offset from the beginning of the
> > extent? Would you oppose to have it updated to return the offset initially
> > requested? Or maybe, change ext4_fiemap() to use iomap_fiemap()?

ext4_fiemap() predates iomap_fiemap().  In fact, it used to be that
all of the file systems had their own fiemap() implementation.   

> > I read the fiemap documentation, but I didn't get a clear understanding if
> > fiemap should be returning the beginning of the extent, the offset initially
> > requested, or if it depends on FS implementation.
> 
> I think the fiemap docs[1] explicitly state that ext4's behavior is valid:
> 
> > Extents returned mirror
> > those on disk - that is, the logical offset of the 1st returned extent
> > may start before fm_start, and the range covered by the last returned
> > extent may end after fm_length.

Actually, I read, "Extents returned mirror those on disk" as meaning
that the ext4 behavior is *mandated* by the docs.  It would be
interesting to see what XFS did before the iomap_fiemap() conversion.
Or it could have been that the docs were inconsistent with what XFS
was doing and then when when ext4_fiemap() was implemented, we
followed the docs.  Some software archeology would be required to know
for sure.

						- Ted