[PATCH] md: better handling of readerrors with raid5.

This patch changes the behaviour of raid5 when it gets a read error.
Instead of just failing the device, it tried to find out what should have
been there, and writes it over the bad block.  For some media-errors, this
has a reasonable chance of fixing the error.  If the write succeeds, and a
subsequent read succeeds as well, raid5 decided the address is OK and
conitnues.

Instead of failing a drive on read-error, we attempt to re-write the block,
and then re-read.  If that all works, we allow the device to remain in the
array.

Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

include/linux/raid/raid5.h

@@ -154,6 +154,8 @@ struct stripe_head {
 #define	R5_Wantwrite	5
 #define	R5_Syncio	6	/* this io need to be accounted as resync io */
 #define	R5_Overlap	7	/* There is a pending overlapping request on this block */
+#define	R5_ReadError	8	/* seen a read error here recently */
+#define	R5_ReWrite	9	/* have tried to over-write the readerror */
 
 /*
  * Write method