dm cache: add policy name to status output

The cache's policy may have been established using the "default" alias,
which is currently the "mq" policy but the default policy may change in
the future. It is useful to know exactly which policy is being used.

Add a 'real' member to the dm_cache_policy_type structure and have the
"default" dm_cache_policy_type point to the real "mq"
dm_cache_policy_type. Update dm_cache_policy_get_name() to check if
real is set, if so report the name of the real policy (not the alias).

Requested-by: Jonathan Brassow <[e-mail]>
Signed-off-by: Mike Snitzer <[e-mail]>

Documentation/device-mapper/cache.txt

···
 <cache block size> <#used cache blocks>/<#total cache blocks>
 <#read hits> <#read misses> <#write hits> <#write misses>
 <#demotions> <#promotions> <#dirty> <#features> <features>*
-<#core args> <core args>* <#policy args> <policy args>*
+<#core args> <core args>* <policy name> <#policy args> <policy args>*
 
 metadata block size	 : Fixed block size for each metadata block in
 			     sectors
···
 #core args		 : Number of core arguments (must be even)
 core args		 : Key/value pairs for tuning the core
 			     e.g. migration_threshold
+policy name		 : Name of the policy
 #policy args		 : Number of policy arguments to follow (must be even)
 policy args		 : Key/value pairs
 			     e.g. sequential_threshold

drivers/md/dm-cache-policy-mq.c

···
 	.version = {1, 2, 0},
 	.hint_size = 4,
 	.owner = THIS_MODULE,
-	.create = mq_create
+	.create = mq_create,
+	.real = &mq_policy_type
 };
 
 static int __init mq_init(void)

drivers/md/dm-cache-policy.c

···
 {
 	struct dm_cache_policy_type *t = p->private;
 
+	/* if t->real is set then an alias was used (e.g. "default") */
+	if (t->real)
+		return t->real->name;
+
 	return t->name;
 }
 EXPORT_SYMBOL_GPL(dm_cache_policy_get_name);

drivers/md/dm-cache-policy.h

···
 	unsigned version[CACHE_POLICY_VERSION_SIZE];
 
 	/*
+	 * For use by an alias dm_cache_policy_type to point to the
+	 * real dm_cache_policy_type.
+	 */
+	struct dm_cache_policy_type *real;
+
+	/*
 	 * Policies may store a hint for each each cache block.
 	 * Currently the size of this hint must be 0 or 4 bytes but we
 	 * expect to relax this in future.

drivers/md/dm-cache-target.c

···
  * <#demotions> <#promotions> <#dirty>
  * <#features> <features>*
  * <#core args> <core args>
- * <#policy args> <policy args>*
+ * <policy name> <#policy args> <policy args>*
  */
 static void cache_status(struct dm_target *ti, status_type_t type,
 			 unsigned status_flags, char *result, unsigned maxlen)
···
 		}
 
 		DMEMIT("2 migration_threshold %llu ", (unsigned long long) cache->migration_threshold);
+
+		DMEMIT("%s ", dm_cache_policy_get_name(cache->policy));
 		if (sz < maxlen) {
 			r = policy_emit_config_values(cache->policy, result + sz, maxlen - sz);
 			if (r)