raid5-ppl: runtime PPL enabling or disabling

Allow writing to 'consistency_policy' attribute when the array is
active. Add a new function 'change_consistency_policy' to the
md_personality operations structure to handle the change in the
personality code. Values "ppl" and "resync" are accepted and
turn PPL on and off respectively.

When enabling PPL its location and size should first be set using
'ppl_sector' and 'ppl_size' attributes and a valid PPL header should be
written at this location on each member device.

Enabling or disabling PPL is performed under a suspended array.  The
raid5_reset_stripe_cache function frees the stripe cache and allocates
it again in order to allocate or free the ppl_pages for the stripes in
the stripe cache.
Signed-off-by: Artur Paszkiewicz <artur.paszkiewicz@intel.com>
Signed-off-by: Shaohua Li <shli@fb.com>

drivers/md/md.c

@@ -4996,14 +4996,20 @@ consistency_policy_show(struct mddev *mddev, char *page)
 static ssize_t
 consistency_policy_store(struct mddev *mddev, const char *buf, size_t len)
 {
+	int err = 0;
+
 	if (mddev->pers) {
-		return -EBUSY;
+		if (mddev->pers->change_consistency_policy)
+			err = mddev->pers->change_consistency_policy(mddev, buf);
+		else
+			err = -EBUSY;
 	} else if (mddev->external && strncmp(buf, "ppl", 3) == 0) {
 		set_bit(MD_HAS_PPL, &mddev->flags);
-		return len;
 	} else {
-		return -EINVAL;
+		err = -EINVAL;
 	}
+
+	return err ? err : len;
 }
 
 static struct md_sysfs_entry md_consistency_policy =

drivers/md/md.h

@@ -545,6 +545,8 @@ struct md_personality
 	/* congested implements bdi.congested_fn().
 	 * Will not be called while array is 'suspended' */
 	int (*congested)(struct mddev *mddev, int bits);
+	/* Changes the consistency policy of an active array. */
+	int (*change_consistency_policy)(struct mddev *mddev, const char *buf);
 };
 
 struct md_sysfs_entry {

drivers/md/raid5-ppl.c

@@ -1187,6 +1187,10 @@ int ppl_init_log(struct r5conf *conf)
 		 */
 		mddev->recovery_cp = MaxSector;
 		set_bit(MD_SB_CHANGE_CLEAN, &mddev->sb_flags);
+	} else if (mddev->pers && ppl_conf->mismatch_count > 0) {
+		/* no mismatch allowed when enabling PPL for a running array */
+		ret = -EINVAL;
+		goto err;
 	}
 
 	conf->log_private = ppl_conf;

drivers/md/raid5.c

@@ -8334,6 +8334,58 @@ static void *raid6_takeover(struct mddev *mddev)
 	return setup_conf(mddev);
 }
 
+static void raid5_reset_stripe_cache(struct mddev *mddev)
+{
+	struct r5conf *conf = mddev->private;
+
+	mutex_lock(&conf->cache_size_mutex);
+	while (conf->max_nr_stripes &&
+	       drop_one_stripe(conf))
+		;
+	while (conf->min_nr_stripes > conf->max_nr_stripes &&
+	       grow_one_stripe(conf, GFP_KERNEL))
+		;
+	mutex_unlock(&conf->cache_size_mutex);
+}
+
+static int raid5_change_consistency_policy(struct mddev *mddev, const char *buf)
+{
+	struct r5conf *conf;
+	int err;
+
+	err = mddev_lock(mddev);
+	if (err)
+		return err;
+	conf = mddev->private;
+	if (!conf) {
+		mddev_unlock(mddev);
+		return -ENODEV;
+	}
+
+	if (strncmp(buf, "ppl", 3) == 0 && !raid5_has_ppl(conf)) {
+		mddev_suspend(mddev);
+		set_bit(MD_HAS_PPL, &mddev->flags);
+		err = log_init(conf, NULL);
+		if (!err)
+			raid5_reset_stripe_cache(mddev);
+		mddev_resume(mddev);
+	} else if (strncmp(buf, "resync", 6) == 0 && raid5_has_ppl(conf)) {
+		mddev_suspend(mddev);
+		log_exit(conf);
+		raid5_reset_stripe_cache(mddev);
+		mddev_resume(mddev);
+	} else {
+		err = -EINVAL;
+	}
+
+	if (!err)
+		md_update_sb(mddev, 1);
+
+	mddev_unlock(mddev);
+
+	return err;
+}
+
 static struct md_personality raid6_personality =
 {
 	.name		= "raid6",
@@ -8379,6 +8431,7 @@ static struct md_personality raid5_personality =
 	.quiesce	= raid5_quiesce,
 	.takeover	= raid5_takeover,
 	.congested	= raid5_congested,
+	.change_consistency_policy = raid5_change_consistency_policy,
 };
 
 static struct md_personality raid4_personality =