md/raid10: avoid reading from known bad blocks - part 1

This patch just covers the basic read path:
 1/ read_balance needs to check for badblocks, and return not only
    the chosen slot, but also how many good blocks are available
    there.
 2/ read submission must be ready to issue multiple reads to
    different devices as different bad blocks on different devices
    could mean that a single large read cannot be served by any one
    device, but can still be served by the array.
    This requires keeping count of the number of outstanding requests
    per bio.  This count is stored in 'bi_phys_segments'

On read error we currently just fail the request if another target
cannot handle the whole request.  Next patch refines that a bit.

Signed-off-by: NeilBrown <neilb@suse.de>

1 files changed, 4 insertions, 0 deletions

diff --git a/drivers/md/raid10.h b/drivers/md/raid10.h
index a485914c48c1..c646152ba4e4 100644
--- a/drivers/md/raid10.h
+++ b/drivers/md/raid10.h
@@ -124,4 +124,8 @@ struct r10bio_s {
 #define	R10BIO_IsSync	1
 #define	R10BIO_IsRecover 2
 #define	R10BIO_Degraded 3
+/* Set ReadError on bios that experience a read error
+ * so that raid10d knows what to do with them.
+ */
+#define	R10BIO_ReadError 4
 #endif