xfs: return errors from partial I/O failures to files

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 in partial-failure cases such as one disk of a RAID0 array being offline.

The issue can occur when there are more than one 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.

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 a failed bio. Also check that ioend->io_error is 0 so that the first error reported will be the error code returned.

---

index 3859f5e..458fced 100644
@@ -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 (!ioend->io_error && !test_bit(BIO_UPTODATE, &bio->bi_flags))
+		ioend->io_error = error;
 
 	/* Toss bio and pass work off to an xfsdatad thread */
 	bio->bi_private = NULL;

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