On Sat, Jun 02, 2018 at 11:28:53PM -0400, Theodore Y. Ts'o wrote: > 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. IIRC the pre-iomap xfs_vn_fiemap implementation only returned extent data for the block range requested. As far as I can tell, the current xfs iomap implementation retains that behavior. The fiemap spec says that "it is valid for an extents [sic] logical offset to start before the request or its logical length to extend past the request". To my eyes, that means either behavior is acceptable. --D > > - Ted
