Re: Ext4 fiemap implementation

From: "Darrick J. Wong" <darrick.wong@oracle.com>
To: "Theodore Y. Ts'o" <tytso@mit.edu>
Cc: Eric Sandeen <sandeen@sandeen.net>,
	Carlos Maiolino <cmaiolino@redhat.com>,
	linux-fsdevel@vger.kernel.org, david@fromorbit.com,
	hch@infradead.org
Subject: Re: Ext4 fiemap implementation
Date: Mon, 4 Jun 2018 09:43:09 -0700	[thread overview]
Message-ID: <20180604164309.GB23842@magnolia> (raw)
In-Reply-To: <20180603032853.GA3585@thunk.org>

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