[PATCH] md: allow dirty raid[456] arrays to be started at boot

See patch to md.txt for more details
Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

Documentation/md.txt

@@ -51,6 +51,30 @@ superblock can be autodetected and run at boot time.
 The kernel parameter "raid=partitionable" (or "raid=part") means
 that all auto-detected arrays are assembled as partitionable.
 
+Boot time assembly of degraded/dirty arrays
+-------------------------------------------
+
+If a raid5 or raid6 array is both dirty and degraded, it could have
+undetectable data corruption.  This is because the fact that it is
+'dirty' means that the parity cannot be trusted, and the fact that it
+is degraded means that some datablocks are missing and cannot reliably
+be reconstructed (due to no parity).
+
+For this reason, md will normally refuse to start such an array.  This
+requires the sysadmin to take action to explicitly start the array
+desipite possible corruption.  This is normally done with
+   mdadm --assemble --force ....
+
+This option is not really available if the array has the root
+filesystem on it.  In order to support this booting from such an
+array, md supports a module parameter "start_dirty_degraded" which,
+when set to 1, bypassed the checks and will allows dirty degraded
+arrays to be started.
+
+So, to boot with a root filesystem of a dirty degraded raid[56], use
+
+   md-mod.start_dirty_degraded=1
+
 
 Superblock formats
 ------------------

drivers/md/md.c

@@ -1937,6 +1937,7 @@ static void md_safemode_timeout(unsigned long data)
 	md_wakeup_thread(mddev->thread);
 }
 
+static int start_dirty_degraded;
 
 static int do_md_run(mddev_t * mddev)
 {
@@ -2048,6 +2049,7 @@ static int do_md_run(mddev_t * mddev)
 	mddev->recovery = 0;
 	mddev->resync_max_sectors = mddev->size << 1; /* may be over-ridden by personality */
 	mddev->barriers_work = 1;
+	mddev->ok_start_degraded = start_dirty_degraded;
 
 	if (start_readonly)
 		mddev->ro = 2; /* read-only, but switch on first write */
@@ -4509,6 +4511,8 @@ static int set_ro(const char *val, struct kernel_param *kp)
 }
 
 module_param_call(start_ro, set_ro, get_ro, NULL, 0600);
+module_param(start_dirty_degraded, int, 0644);
+
 
 EXPORT_SYMBOL(register_md_personality);
 EXPORT_SYMBOL(unregister_md_personality);

drivers/md/raid5.c

@@ -1904,11 +1904,18 @@ static int run(mddev_t *mddev)
 
 	if (mddev->degraded == 1 &&
 	    mddev->recovery_cp != MaxSector) {
+		if (mddev->ok_start_degraded)
+			printk(KERN_WARNING
+			       "raid5: starting dirty degraded array: %s"
+			       "- data corruption possible.\n",
+			       mdname(mddev));
+		else {
 			printk(KERN_ERR
 			       "raid5: cannot start dirty degraded array for %s\n",
 			       mdname(mddev));
 			goto abort;
 		}
+	}
 
 	{
 		mddev->thread = md_register_thread(raid5d, mddev, "%s_raid5");

drivers/md/raid6main.c

@@ -1929,13 +1929,18 @@ static int run(mddev_t *mddev)
 		goto abort;
 	}
 
-#if 0				/* FIX: For now */
 	if (mddev->degraded > 0 &&
 	    mddev->recovery_cp != MaxSector) {
-		printk(KERN_ERR "raid6: cannot start dirty degraded array for %s\n", mdname(mddev));
+		if (mddev->ok_start_degraded)
+			printk(KERN_WARNING "raid6: starting dirty degraded array:%s"
+			       "- data corruption possible.\n",
+			       mdname(mddev));
+		else {
+			printk(KERN_ERR "raid6: cannot start dirty degraded array"
+			       " for %s\n", mdname(mddev));
 			goto abort;
 		}
-#endif
+	}
 
 	{
 		mddev->thread = md_register_thread(raid6d, mddev, "%s_raid6");

include/linux/raid/md_k.h

@@ -183,6 +183,7 @@ struct mddev_s
 	sector_t			resync_mismatches; /* count of sectors where
 							    * parity/replica mismatch found
 							    */
+	int				ok_start_degraded;
 	/* recovery/resync flags 
 	 * NEEDED:   we might need to start a resync/recover
 	 * RUNNING:  a thread is running, or about to be started