On Sat, Jun 09, 2018 at 12:41:26AM +0200, Mark Fasheh wrote: > Hi Darrick, > > On Mon, Jun 04, 2018 at 09:43:09AM -0700, Darrick J. Wong wrote: > > 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. > > You have to take the whole paragraph (well the first half) together: > > "All offsets and lengths are in bytes and mirror those on disk. It is valid > for an extents logical offset to start before the request or its logical > length to extend past the request." > > So in other words, mirror what's on disk. That might mean that > the returned extent might have a logical start before what the user > requested. The length might be past the request too, again because we're > mirroring what's on disk. > > In fact, at no point is it specified that the fs can move the logical > start of the returned extent *forward*. The text is quite explicit that the > logcal end can only be *before* the request because that's the only way that > 'mirror what's on disk' can work for the user. > > Thanks, > --Mark > > Btw, I realize the original e-mail was about physical offset but for the > purposes of this conversation the two values are mathematically linked. So, you are saying iomap implementation violates FIEMAP specs? -- Carlos
