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From: Carlos Maiolino <cmaiolino@redhat.com>
To: Mark Fasheh <mfasheh@suse.de>
Cc: "Darrick J. Wong" <darrick.wong@oracle.com>,
	"Theodore Y. Ts'o" <tytso@mit.edu>,
	Eric Sandeen <sandeen@sandeen.net>,
	linux-fsdevel@vger.kernel.org, david@fromorbit.com,
	hch@infradead.org
Subject: Re: Ext4 fiemap implementation
Date: Mon, 11 Jun 2018 09:28:27 +0200	[thread overview]
Message-ID: <20180611072827.arsyq56m7ptnedig@odin.usersys.redhat.com> (raw)
In-Reply-To: <20180608224126.GD28053@wotan.suse.de>

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

  reply	other threads:[~2018-06-11  7:28 UTC|newest]

Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-06-01 12:36 Carlos Maiolino
2018-06-01 15:01 ` Eric Sandeen
2018-06-03  3:28   ` Theodore Y. Ts'o
2018-06-04 16:43     ` Darrick J. Wong
2018-06-06 13:13       ` Carlos Maiolino
2018-06-06 14:40         ` Darrick J. Wong
2018-06-07  8:31           ` Carlos Maiolino
2018-06-07 16:25             ` Darrick J. Wong
2018-06-08  8:18               ` Carlos Maiolino
2018-06-08 22:41       ` Mark Fasheh
2018-06-11  7:28         ` Carlos Maiolino [this message]
2018-06-12 23:52           ` Mark Fasheh
2018-06-13  3:06             ` Theodore Y. Ts'o
2018-06-13  3:32               ` Dave Chinner
2018-06-13  5:04                 ` Theodore Y. Ts'o
2018-06-13  7:41                   ` Dave Chinner
2018-06-13 12:09                     ` Christoph Hellwig
2018-06-14  8:14                       ` Carlos Maiolino
2018-06-01 18:57 ` Andreas Dilger

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