dm cache: speed up writing of the hint array

It's far quicker to always delete the hint array and recreate with
dm_array_new() because we avoid the copying caused by mutation.

Also simplifies the policy interface, replacing the walk_hints() with
the simpler get_hint().
Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-cache-metadata.c

@@ -1368,10 +1368,24 @@ int dm_cache_get_metadata_dev_size(struct dm_cache_metadata *cmd,
 
 /*----------------------------------------------------------------*/
 
-static int begin_hints(struct dm_cache_metadata *cmd, struct dm_cache_policy *policy)
+static int get_hint(uint32_t index, void *value_le, void *context)
+{
+	uint32_t value;
+	struct dm_cache_policy *policy = context;
+
+	value = policy_get_hint(policy, to_cblock(index));
+	*((__le32 *) value_le) = cpu_to_le32(value);
+
+	return 0;
+}
+
+/*
+ * It's quicker to always delete the hint array, and recreate with
+ * dm_array_new().
+ */
+static int write_hints(struct dm_cache_metadata *cmd, struct dm_cache_policy *policy)
 {
 	int r;
-	__le32 value;
 	size_t hint_size;
 	const char *policy_name = dm_cache_policy_get_name(policy);
 	const unsigned *policy_version = dm_cache_policy_get_version(policy);
@@ -1380,63 +1394,23 @@ static int begin_hints(struct dm_cache_metadata *cmd, struct dm_cache_policy *po
 	    (strlen(policy_name) > sizeof(cmd->policy_name) - 1))
 		return -EINVAL;
 
-	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));
+	strncpy(cmd->policy_name, policy_name, sizeof(cmd->policy_name));
+	memcpy(cmd->policy_version, policy_version, sizeof(cmd->policy_version));
 
-		hint_size = dm_cache_policy_get_hint_size(policy);
-		if (!hint_size)
-			return 0; /* short-circuit hints initialization */
-		cmd->policy_hint_size = hint_size;
+	hint_size = dm_cache_policy_get_hint_size(policy);
+	if (!hint_size)
+		return 0; /* short-circuit hints initialization */
+	cmd->policy_hint_size = hint_size;
 
-		if (cmd->hint_root) {
-			r = dm_array_del(&cmd->hint_info, cmd->hint_root);
-			if (r)
-				return r;
-		}
-
-		r = dm_array_empty(&cmd->hint_info, &cmd->hint_root);
+	if (cmd->hint_root) {
+		r = dm_array_del(&cmd->hint_info, cmd->hint_root);
 		if (r)
 			return r;
-
-		value = cpu_to_le32(0);
-		__dm_bless_for_disk(&value);
-		r = dm_array_resize(&cmd->hint_info, cmd->hint_root, 0,
-				    from_cblock(cmd->cache_blocks),
-				    &value, &cmd->hint_root);
-		if (r)
-			return r;
-	}
-
-	return 0;
-}
-
-static int save_hint(void *context, dm_cblock_t cblock, dm_oblock_t oblock, uint32_t hint)
-{
-	struct dm_cache_metadata *cmd = context;
-	__le32 value = cpu_to_le32(hint);
-	int r;
-
-	__dm_bless_for_disk(&value);
-
-	r = dm_array_set_value(&cmd->hint_info, cmd->hint_root,
-			       from_cblock(cblock), &value, &cmd->hint_root);
-	cmd->changed = true;
-
-	return r;
-}
-
-static int write_hints(struct dm_cache_metadata *cmd, struct dm_cache_policy *policy)
-{
-	int r;
-
-	r = begin_hints(cmd, policy);
-	if (r) {
-		DMERR("begin_hints failed");
-		return r;
 	}
 
-	return policy_walk_mappings(policy, save_hint, cmd);
+	return dm_array_new(&cmd->hint_info, &cmd->hint_root,
+			    from_cblock(cmd->cache_blocks),
+			    get_hint, policy);
 }
 
 int dm_cache_write_hints(struct dm_cache_metadata *cmd, struct dm_cache_policy *policy)

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

@@ -395,7 +395,7 @@ static void init_policy_functions(struct policy *p)
 	p->policy.set_dirty = wb_set_dirty;
 	p->policy.clear_dirty = wb_clear_dirty;
 	p->policy.load_mapping = wb_load_mapping;
-	p->policy.walk_mappings = NULL;
+	p->policy.get_hint = NULL;
 	p->policy.remove_mapping = wb_remove_mapping;
 	p->policy.writeback_work = wb_writeback_work;
 	p->policy.force_mapping = wb_force_mapping;

drivers/md/dm-cache-policy-internal.h

@@ -48,10 +48,10 @@ static inline int policy_load_mapping(struct dm_cache_policy *p,
 	return p->load_mapping(p, oblock, cblock, hint, hint_valid);
 }
 
-static inline int policy_walk_mappings(struct dm_cache_policy *p,
-				      policy_walk_fn fn, void *context)
+static inline uint32_t policy_get_hint(struct dm_cache_policy *p,
+				       dm_cblock_t cblock)
 {
-	return p->walk_mappings ? p->walk_mappings(p, fn, context) : 0;
+	return p->get_hint ? p->get_hint(p, cblock) : 0;
 }
 
 static inline int policy_writeback_work(struct dm_cache_policy *p,

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

@@ -1375,41 +1375,15 @@ static int smq_load_mapping(struct dm_cache_policy *p,
 	return 0;
 }
 
-static int smq_save_hints(struct smq_policy *mq, struct queue *q,
-			  policy_walk_fn fn, void *context)
-{
-	int r;
-	unsigned level;
-	struct entry *e;
-
-	for (level = 0; level < q->nr_levels; level++)
-		for (e = l_head(q->es, q->qs + level); e; e = l_next(q->es, e)) {
-			if (!e->sentinel) {
-				r = fn(context, infer_cblock(mq, e),
-				       e->oblock, e->level);
-				if (r)
-					return r;
-			}
-		}
-
-	return 0;
-}
-
-static int smq_walk_mappings(struct dm_cache_policy *p, policy_walk_fn fn,
-			     void *context)
+static uint32_t smq_get_hint(struct dm_cache_policy *p, dm_cblock_t cblock)
 {
 	struct smq_policy *mq = to_smq_policy(p);
-	int r = 0;
+	struct entry *e = get_entry(&mq->cache_alloc, from_cblock(cblock));
 
-	/*
-	 * We don't need to lock here since this method is only called once
-	 * the IO has stopped.
-	 */
-	r = smq_save_hints(mq, &mq->clean, fn, context);
-	if (!r)
-		r = smq_save_hints(mq, &mq->dirty, fn, context);
+	if (!e->allocated)
+		return 0;
 
-	return r;
+	return e->level;
 }
 
 static void __remove_mapping(struct smq_policy *mq, dm_oblock_t oblock)
@@ -1616,7 +1590,7 @@ static void init_policy_functions(struct smq_policy *mq, bool mimic_mq)
 	mq->policy.set_dirty = smq_set_dirty;
 	mq->policy.clear_dirty = smq_clear_dirty;
 	mq->policy.load_mapping = smq_load_mapping;
-	mq->policy.walk_mappings = smq_walk_mappings;
+	mq->policy.get_hint = smq_get_hint;
 	mq->policy.remove_mapping = smq_remove_mapping;
 	mq->policy.remove_cblock = smq_remove_cblock;
 	mq->policy.writeback_work = smq_writeback_work;

drivers/md/dm-cache-policy.h

@@ -90,9 +90,6 @@ struct policy_result {
 	dm_cblock_t cblock;	/* POLICY_HIT, POLICY_NEW, POLICY_REPLACE */
 };
 
-typedef int (*policy_walk_fn)(void *context, dm_cblock_t cblock,
-			      dm_oblock_t oblock, uint32_t hint);
-
 /*
  * The cache policy object.  Just a bunch of methods.  It is envisaged that
  * this structure will be embedded in a bigger, policy specific structure
@@ -158,8 +155,11 @@ struct dm_cache_policy {
 	int (*load_mapping)(struct dm_cache_policy *p, dm_oblock_t oblock,
 			    dm_cblock_t cblock, uint32_t hint, bool hint_valid);
 
-	int (*walk_mappings)(struct dm_cache_policy *p, policy_walk_fn fn,
-			     void *context);
+	/*
+	 * Gets the hint for a given cblock.  Called in a single threaded
+	 * context.  So no locking required.
+	 */
+	uint32_t (*get_hint)(struct dm_cache_policy *p, dm_cblock_t cblock);
 
 	/*
 	 * Override functions used on the error paths of the core target.