On 08/26/2015 06:19 PM, Dave Chinner wrote:

On Wed, Aug 26, 2015 at 03:06:36PM -0400, David Jeffery wrote:

quoted

There is an issue with xfs's error reporting in some cases of I/O partially
failing and partially succeeding. Calls like fsync() can report success even
though not all I/O was successful.

Hi David,

I read your bug report last night and after considering all the work
you put into it, I was going to ask if you wanted to finish off the
job by writing the patch to fix it. But you beat me to it.

Nice work! :)

quoted

The issue can occur when there are multiple bio per xfs_ioend struct.
Each call to xfs_end_bio() for a bio completing will write a value to
ioend->io_error.  If a successful bio completes after any failed bio, no
error is reported do to it writing 0 over the error code set by any failed bio.
The I/O error information is now lost and when the ioend is completed
only success is reported back up the filesystem stack.

It's worth mentioning the case that this was seen in - a single
failed disk in a raid 0 stripe, and the error from the bio to the
failed disk was overwritten by the successes from the bios to the
other disks.

FWIW, I think that we also need to create an xfstest for this case,
too, because it's clear that this is a big hole in our test coverage
(i.e.  partial block device failure). It might be best to talk to
Eryu (cc'd) to get your reproducer converted into a xfstest case
that we can then test all filesystems against?

quoted

xfs_end_bio() should only set ioend->io_error in the case of BIO_UPTODATE
being clear.  ioend->io_error is initialized to 0 at allocation so only needs
to be updated by any failed bio structs. This ensures an error can be reported
to the application.

Signed-off-by: David Jeffery <redacted>
---

Best to add a "cc: [off-list ref]" so that it gets pushed
back to all the stable kernels automatically which it hits Linus'
tree.

One minor change to the fix:

quoted

diff --git a/fs/xfs/xfs_aops.c b/fs/xfs/xfs_aops.c
index 3859f5e..b82b128 100644
--- a/fs/xfs/xfs_aops.c
+++ b/fs/xfs/xfs_aops.c

@@ -356,7 +356,8 @@ xfs_end_bio(
 {
 	xfs_ioend_t		*ioend = bio->bi_private;
 
-	ioend->io_error = test_bit(BIO_UPTODATE, &bio->bi_flags) ? 0 : error;
+	if (!test_bit(BIO_UPTODATE, &bio->bi_flags))
+		ioend->io_error = error;

We should preserve the original error that was reported, rather than
report the last one. ioend->io_error is always initialised to zero,
so we can simply do:

Is there a particular reason to prefer the first error?  Is it just
standard practice?  I originally made a version which preserved the
first error but couldn't think of a reason why the first or last would
be best.  So I went with the patch which has the simpler if statement.

	if (!ioend->io_error && !test_bit(BIO_UPTODATE, &bio->bi_flags))
		ioend->io_error = error;

Can you update the patch and resend it? I've got a couple of other
fixes that I need to push to the for-next tree in the next couple of
days (i.e. before the 4.3. merge window opens) and I'd like to get
this one in as well.

Cheers,

Dave

I'll get a new version sent.

David Jeffery

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