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From: "Darrick J. Wong" <darrick.wong@oracle.com>
To: Carlos Maiolino <cmaiolino@redhat.com>
Cc: "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: Thu, 7 Jun 2018 09:25:04 -0700	[thread overview]
Message-ID: <20180607162504.GD9426@magnolia> (raw)
In-Reply-To: <20180607083101.bqazknwqswybvxpm@odin.usersys.redhat.com>

On Thu, Jun 07, 2018 at 10:31:01AM +0200, Carlos Maiolino wrote:
> On Wed, Jun 06, 2018 at 07:40:18AM -0700, Darrick J. Wong wrote:
> > On Wed, Jun 06, 2018 at 03:13:39PM +0200, Carlos Maiolino wrote:
> > > Sigh..
> > > 
> > > 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.
> > > > 
> > > 
> > > Ok, thanks for the input everyone. I believe Eric's idea then is the one which
> > > makes more sense. If both behaviors are valid, to make fiemap() usage for
> > > fibmap, I think I'll need to get the extent returned by the filesystem and look
> > > for the block into the extent. Thanks a lot for the ideas.
> > 
> > Just to throw another monkey wrench into the machine, have you
> > considered using iomap_bmap() instead?  It's new for 4.18...
> 
> No, but thanks, I'll look into it.

You might also look at converting ext4 to use iomap_swapfile_activate
since that's new too.  iomap_bmap won't report blocks for unwritten
extents (because we shouldn't be pointing userspace at stale disk
blocks), which breaks swapfiles with unwritten extents.

--D

> 
> > 
> > --D
> > 
> > > 
> > > 
> > > -- 
> > > Carlos
> 
> -- 
> Carlos

  reply	other threads:[~2018-06-07 16:25 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 [this message]
2018-06-08  8:18               ` Carlos Maiolino
2018-06-08 22:41       ` Mark Fasheh
2018-06-11  7:28         ` Carlos Maiolino
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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