On Wed, Jul 13, 2016 at 03:19:38PM +0200, David Sterba wrote:

On Tue, Jul 12, 2016 at 10:26:43PM -0700, Darrick J. Wong wrote:

quoted

On Mon, Jul 11, 2016 at 05:35:37PM -0700, Omar Sandoval wrote:

quoted

Hey, Darrick,

generic/182 is failing on Btrfs for me with the following output:

--- tests/generic/182.out       2016-07-07 19:51:54.000000000 -0700
+++ /tmp/fixxfstests/xfstests/results//generic/182.out.bad      2016-07-11 17:28:28.230039216 -0700

@@ -1,12 +1,10 @@
 QA output created by 182
 Create the original files
-dedupe: Extents did not match.
 f4820540fc0ac02750739896fe028d56  TEST_DIR/test-182/file1
 69ad53078a16243d98e21d9f8704a071  TEST_DIR/test-182/file2
 69ad53078a16243d98e21d9f8704a071  TEST_DIR/test-182/file2.chk
 Compare against check files
 Make the original file almost dedup-able
-dedupe: Extents did not match.
 f4820540fc0ac02750739896fe028d56  TEST_DIR/test-182/file1
 158d4e3578b94b89cbb44493a2110fb9  TEST_DIR/test-182/file2
 158d4e3578b94b89cbb44493a2110fb9  TEST_DIR/test-182/file2.chk

It looks like that test is checking that a dedupe with length == 0 is
treated as a dedupe to EOF, but Btrfs doesn't do that [1]. As far as I
can tell, it never did, but maybe I'm just confused. What was the
behavior when you introduced that test? That seems like a reasonable
thing to do, but I wanted to clear this up before changing/fixing Btrfs.

It's a shortcut that we're introducing in the upcoming XFS implementation,
since it shares the same back end as clone/clonerange, which both have
this behavior.

The support for zero length does not seem to be mentioned anywhere with
the dedupe range ioctl [1], so the current implemetnation is "up to
spec". That it should be valid is hidden in clone_verify_area where a
zero length is substituted with OFFSET_MAX

http://lxr.free-electrons.com/source/fs/read_write.c#L1607

So it looks like it's up to the implementation in the filesystem to
handle that. As the btrfs ioctl was extent-based, a zero length extent
does not make sense, so this case was not handled. But in your patch

2b3909f8a7fe94e0234850aa9d120cca15b6e1f7
btrfs: use new dedupe data function pointer

it was suddenly expected to work. So the missing bits are either 'not
supported' for zero length or actually implement iteration over the
whole file.

[1] https://www.mankier.com/2/ioctl_fideduperange

Well, we can't change the semantics now because there could be programs that
aren't expecting a nonzero return from a length == 0 dedupe, so like Christoph
said, I'll just change generic/182 and make the VFS wrapper emulate the btrfs
behavior so that any subsequent implementation won't hit this.

I'll update the clone/clonerange manpages to mention the 0 -> EOF behavior.
Would've been awesome if anyone had bothered to document the damn ioctls
and write tests to call out these subtleties rather than leave the next
implementer to clean up all the technical debt.  I dunno, maybe there /is/
a secret cache of btrfs regression tests somewhere that I haven't found.
<grumble>

--D