Re: Ext4 fiemap implementation

[Mark Fasheh <mfasheh@suse.de>]

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.