dm cache: policy ignore hints if generated by different version

When reading the dm cache metadata from disk, ignore the policy hints
unless they were generated by the same major version number of the same
policy module.

The hints are considered to be private data belonging to the specific
module that generated them and there is no requirement for them to make
sense to different versions of the policy that generated them.
Policy modules are all required to work fine if no previous hints are
supplied (or if existing hints are lost).

Signed-off-by: Mike Snitzer <[e-mail]>
Signed-off-by: Alasdair G Kergon <[e-mail]>

drivers/md/dm-cache-metadata.c

···
 	bool hints_valid;
 };
 
+static bool policy_unchanged(struct dm_cache_metadata *cmd,
+			     struct dm_cache_policy *policy)
+{
+	const char *policy_name = dm_cache_policy_get_name(policy);
+	const unsigned *policy_version = dm_cache_policy_get_version(policy);
+	size_t policy_hint_size = dm_cache_policy_get_hint_size(policy);
+
+	/*
+	 * Ensure policy names match.
+	 */
+	if (strncmp(cmd->policy_name, policy_name, sizeof(cmd->policy_name)))
+		return false;
+
+	/*
+	 * Ensure policy major versions match.
+	 */
+	if (cmd->policy_version[0] != policy_version[0])
+		return false;
+
+	/*
+	 * Ensure policy hint sizes match.
+	 */
+	if (cmd->policy_hint_size != policy_hint_size)
+		return false;
+
+	return true;
+}
+
 static bool hints_array_initialized(struct dm_cache_metadata *cmd)
 {
 	return cmd->hint_root && cmd->policy_hint_size;
 }
 
 static bool hints_array_available(struct dm_cache_metadata *cmd,
-				  const char *policy_name)
+				  struct dm_cache_policy *policy)
 {
-	bool policy_names_match = !strncmp(cmd->policy_name, policy_name,
-					   sizeof(cmd->policy_name));
-
-	return cmd->clean_when_opened && policy_names_match &&
+	return cmd->clean_when_opened && policy_unchanged(cmd, policy) &&
 		hints_array_initialized(cmd);
 }
 
···
 	return r;
 }
 
-static int __load_mappings(struct dm_cache_metadata *cmd, const char *policy_name,
+static int __load_mappings(struct dm_cache_metadata *cmd,
+			   struct dm_cache_policy *policy,
 			   load_mapping_fn fn, void *context)
 {
 	struct thunk thunk;
···
 
 	thunk.cmd = cmd;
 	thunk.respect_dirty_flags = cmd->clean_when_opened;
-	thunk.hints_valid = hints_array_available(cmd, policy_name);
+	thunk.hints_valid = hints_array_available(cmd, policy);
 
 	return dm_array_walk(&cmd->info, cmd->root, __load_mapping, &thunk);
 }
 
-int dm_cache_load_mappings(struct dm_cache_metadata *cmd, const char *policy_name,
+int dm_cache_load_mappings(struct dm_cache_metadata *cmd,
+			   struct dm_cache_policy *policy,
 			   load_mapping_fn fn, void *context)
 {
 	int r;
 
 	down_read(&cmd->root_lock);
-	r = __load_mappings(cmd, policy_name, fn, context);
+	r = __load_mappings(cmd, policy, fn, context);
 	up_read(&cmd->root_lock);
 
 	return r;
···
 	    (strlen(policy_name) > sizeof(cmd->policy_name) - 1))
 		return -EINVAL;
 
-	if (strcmp(cmd->policy_name, policy_name)) {
+	if (!policy_unchanged(cmd, policy)) {
 		strncpy(cmd->policy_name, policy_name, sizeof(cmd->policy_name));
 		memcpy(cmd->policy_version, policy_version, sizeof(cmd->policy_version));
 

drivers/md/dm-cache-metadata.h

···
 			       dm_cblock_t cblock, bool dirty,
 			       uint32_t hint, bool hint_valid);
 int dm_cache_load_mappings(struct dm_cache_metadata *cmd,
-			   const char *policy_name,
+			   struct dm_cache_policy *policy,
 			   load_mapping_fn fn,
 			   void *context);
 

drivers/md/dm-cache-target.c

···
 	}
 
 	if (!cache->loaded_mappings) {
-		r = dm_cache_load_mappings(cache->cmd,
-					   dm_cache_policy_get_name(cache->policy),
+		r = dm_cache_load_mappings(cache->cmd, cache->policy,
 					   load_mapping, cache);
 		if (r) {
 			DMERR("could not load cache mappings");