[PATCH] md: Core of raid5 resize process

This patch provides the core of the resize/expand process.

sync_request notices if a 'reshape' is happening and acts accordingly.

It allocated new stripe_heads for the next chunk-wide-stripe in the target
geometry, marking them STRIPE_EXPANDING.

Then it finds which stripe heads in the old geometry can provide data needed
by these and marks them STRIPE_EXPAND_SOURCE.  This causes stripe_handle to
read all blocks on those stripes.

Once all blocks on a STRIPE_EXPAND_SOURCE stripe_head are read, any that are
needed are copied into the corresponding STRIPE_EXPANDING stripe_head.  Once a
STRIPE_EXPANDING stripe_head is full, it is marks STRIPE_EXPAND_READY and then
is written out and released.

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

@@ -157,6 +157,7 @@ struct stripe_head {
 #define	R5_ReadError	8	/* seen a read error here recently */
 #define	R5_ReWrite	9	/* have tried to over-write the readerror */
 
+#define	R5_Expanded	10	/* This block now has post-expand data */
 /*
  * Write method
  */
@@ -176,7 +177,8 @@ struct stripe_head {
 #define	STRIPE_DEGRADED		7
 #define	STRIPE_BIT_DELAY	8
 #define	STRIPE_EXPANDING	9
-
+#define	STRIPE_EXPAND_SOURCE	10
+#define	STRIPE_EXPAND_READY	11
 /*
  * Plugging:
  *