3 files changed, 52 insertions, 1 deletions

diff --git a/fs/btrfs/sysfs.c b/fs/btrfs/sysfs.c
index e155b7ce1ee5..5211d13d73f8 100644
--- a/fs/btrfs/sysfs.c
+++ b/fs/btrfs/sysfs.c
@@ -1309,6 +1309,7 @@ static const char *btrfs_read_policy_name[] = {
 	"pid",
 #ifdef CONFIG_BTRFS_EXPERIMENTAL
 	"round-robin",
+	"devid",
 #endif
 };
 
@@ -1364,8 +1365,11 @@ static ssize_t btrfs_read_policy_show(struct kobject *kobj,
 		if (i == BTRFS_READ_POLICY_RR)
 			ret += sysfs_emit_at(buf, ret, ":%u",
 					     READ_ONCE(fs_devices->rr_min_contig_read));
-#endif
 
+		if (i == BTRFS_READ_POLICY_DEVID)
+			ret += sysfs_emit_at(buf, ret, ":%llu",
+					     READ_ONCE(fs_devices->read_devid));
+#endif
 		if (i == policy)
 			ret += sysfs_emit_at(buf, ret, "]");
 	}
@@ -1421,6 +1425,31 @@ static ssize_t btrfs_read_policy_store(struct kobject *kobj,
 
 		return len;
 	}
+
+	if (index == BTRFS_READ_POLICY_DEVID) {
+		if (value != -1) {
+			BTRFS_DEV_LOOKUP_ARGS(args);
+
+			/* Validate input devid. */
+			args.devid = value;
+			if (btrfs_find_device(fs_devices, &args) == NULL)
+				return -EINVAL;
+		} else {
+			/* Set default devid to the devid of the latest device. */
+			value = fs_devices->latest_dev->devid;
+		}
+
+		if (index != READ_ONCE(fs_devices->read_policy) ||
+		    value != READ_ONCE(fs_devices->read_devid)) {
+			WRITE_ONCE(fs_devices->read_policy, index);
+			WRITE_ONCE(fs_devices->read_devid, value);
+
+			btrfs_info(fs_devices->fs_info, "read policy set to '%s:%llu'",
+				   btrfs_read_policy_name[index], value);
+		}
+
+		return len;
+	}
 #endif
 	if (index != READ_ONCE(fs_devices->read_policy)) {
 		WRITE_ONCE(fs_devices->read_policy, index);
diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index cfe1d5ada5f2..b5fd1aa45c4c 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -1331,6 +1331,7 @@ static int open_fs_devices(struct btrfs_fs_devices *fs_devices,
 	fs_devices->read_policy = BTRFS_READ_POLICY_PID;
 #ifdef CONFIG_BTRFS_EXPERIMENTAL
 	fs_devices->rr_min_contig_read = BTRFS_DEFAULT_RR_MIN_CONTIG_READ;
+	fs_devices->read_devid = latest_dev->devid;
 #endif
 
 	return 0;
@@ -5957,6 +5958,19 @@ unsigned long btrfs_full_stripe_len(struct btrfs_fs_info *fs_info,
 }
 
 #ifdef CONFIG_BTRFS_EXPERIMENTAL
+static int btrfs_read_preferred(struct btrfs_chunk_map *map, int first, int num_stripes)
+{
+	for (int index = first; index < first + num_stripes; index++) {
+		const struct btrfs_device *device = map->stripes[index].dev;
+
+		if (device->devid == READ_ONCE(device->fs_devices->read_devid))
+			return index;
+	}
+
+	/* If no read-preferred device is set use the first stripe. */
+	return first;
+}
+
 struct stripe_mirror {
 	u64 devid;
 	int num;
@@ -6046,6 +6060,9 @@ static int find_live_mirror(struct btrfs_fs_info *fs_info,
 	case BTRFS_READ_POLICY_RR:
 		preferred_mirror = btrfs_read_rr(map, first, num_stripes);
 		break;
+	case BTRFS_READ_POLICY_DEVID:
+		preferred_mirror = btrfs_read_preferred(map, first, num_stripes);
+		break;
 #endif
 	}
 
diff --git a/fs/btrfs/volumes.h b/fs/btrfs/volumes.h
index f9fe698a9b4b..120f65e21eeb 100644
--- a/fs/btrfs/volumes.h
+++ b/fs/btrfs/volumes.h
@@ -309,6 +309,8 @@ enum btrfs_read_policy {
 #ifdef CONFIG_BTRFS_EXPERIMENTAL
 	/* Balancing RAID1 reads across all striped devices (round-robin). */
 	BTRFS_READ_POLICY_RR,
+	/* Read from a specific device. */
+	BTRFS_READ_POLICY_DEVID,
 #endif
 	BTRFS_NR_READ_POLICY,
 };
@@ -446,6 +448,9 @@ struct btrfs_fs_devices {
 	 */
 	u32 rr_min_contig_read;
 
+	/* Device to be used for reading in case of RAID1. */
+	u64 read_devid;
+
 	/* Checksum mode - offload it or do it synchronously. */
 	enum btrfs_offload_csum_mode offload_csum_mode;
 #endif