[PATCH v2 3/3] btrfs: add RAID1 preferred read device

[Anand Jain <anand.jain@oracle.com>]

When there's stale data on a mirrored device, this feature lets you choose
which device to read from. Mainly used for testing.

echo "devid:2" > /sys/fs/btrfs/<UUID>/read_policy

Signed-off-by: Anand Jain <anand.jain@oracle.com>
---
 fs/btrfs/disk-io.c |  1 +
 fs/btrfs/sysfs.c   | 32 ++++++++++++++++++++++++++++++--
 fs/btrfs/volumes.c | 20 ++++++++++++++++++++
 fs/btrfs/volumes.h |  5 +++++
 4 files changed, 56 insertions(+), 2 deletions(-)

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 5b157f407e0a..0e7b29282136 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -3322,6 +3322,7 @@ int __cold open_ctree(struct super_block *sb, struct btrfs_fs_devices *fs_device
 	fs_info->sectorsize = sectorsize;
 #ifdef CONFIG_BTRFS_EXPERIMENTAL
 	fs_info->fs_devices->min_contiguous_read = sectorsize;
+	fs_info->fs_devices->read_devid = fs_info->fs_devices->latest_dev->devid;
 #endif
 	fs_info->sectorsize_bits = ilog2(sectorsize);
 	fs_info->sectors_per_page = (PAGE_SIZE >> fs_info->sectorsize_bits);
diff --git a/fs/btrfs/sysfs.c b/fs/btrfs/sysfs.c
index 9f506d46a94c..aa4c9cbaa61f 100644
--- a/fs/btrfs/sysfs.c
+++ b/fs/btrfs/sysfs.c
@@ -1306,7 +1306,7 @@ static ssize_t btrfs_temp_fsid_show(struct kobject *kobj,
 BTRFS_ATTR(, temp_fsid, btrfs_temp_fsid_show);
 
 #ifdef CONFIG_BTRFS_EXPERIMENTAL
-static const char * const btrfs_read_policy_name[] = { "pid", "rotation", "latency" };
+static const char * const btrfs_read_policy_name[] = { "pid", "rotation", "latency", "devid" };
 #else
 static const char * const btrfs_read_policy_name[] = { "pid" };
 #endif
@@ -1332,8 +1332,11 @@ static ssize_t btrfs_read_policy_show(struct kobject *kobj,
 		if (i == BTRFS_READ_POLICY_ROTATION)
 			ret += sysfs_emit_at(buf, ret, ":%d",
 					     fs_devices->min_contiguous_read);
-#endif
 
+		if (i == BTRFS_READ_POLICY_DEVID)
+			ret += sysfs_emit_at(buf, ret, ":%llu",
+							fs_devices->read_devid);
+#endif
 		if (i == policy)
 			ret += sysfs_emit_at(buf, ret, "]");
 	}
@@ -1401,7 +1404,32 @@ static ssize_t btrfs_read_policy_store(struct kobject *kobj,
 
 		return len;
 	}
+
+	if (index == BTRFS_READ_POLICY_DEVID) {
+		u64 value_devid;
+		BTRFS_DEV_LOOKUP_ARGS(args);
+
+		if (value == NULL || kstrtou64(value, 10, &value_devid))
+			return -EINVAL;
+
+		args.devid = value_devid;
+		if (btrfs_find_device(fs_devices, &args) == NULL)
+			return -EINVAL;
+
+		if (index != READ_ONCE(fs_devices->read_policy) ||
+		    (value_devid != READ_ONCE(fs_devices->read_devid))) {
+			WRITE_ONCE(fs_devices->read_policy, index);
+			WRITE_ONCE(fs_devices->read_devid, value_devid);
+
+			btrfs_info(fs_devices->fs_info, "read policy set to '%s:%llu'",
+				   btrfs_read_policy_name[index], value_devid);
+
+		}
+
+		return len;
+	}
 #endif
+
 	if (index != READ_ONCE(fs_devices->read_policy)) {
 		WRITE_ONCE(fs_devices->read_policy, index);
 		btrfs_info(fs_devices->fs_info, "read policy set to '%s'",
diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 8912ee1d8b54..87a072fa9be4 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -5966,6 +5966,23 @@ 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_stripe)
+{
+	int last = first + num_stripe;
+	int stripe_index;
+
+	for (stripe_index = first; stripe_index < last; stripe_index++) {
+		struct btrfs_device *device = map->stripes[stripe_index].dev;
+
+		if (device->devid == READ_ONCE(device->fs_devices->read_devid))
+			return stripe_index;
+	}
+
+	/* If no read-preferred device, use first stripe */
+	return first;
+}
+
 static int btrfs_best_stripe(struct btrfs_fs_info *fs_info,
 			     struct btrfs_chunk_map *map, int first,
 			     int num_stripe)
@@ -6079,6 +6096,9 @@ static int find_live_mirror(struct btrfs_fs_info *fs_info,
 		preferred_mirror = btrfs_best_stripe(fs_info, 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 f9c744b87b61..b5ade9d41fe7 100644
--- a/fs/btrfs/volumes.h
+++ b/fs/btrfs/volumes.h
@@ -308,6 +308,8 @@ enum btrfs_read_policy {
 	BTRFS_READ_POLICY_ROTATION,
 	/* Use the lowest-latency device dynamically */
 	BTRFS_READ_POLICY_LATENCY,
+	/* Read from the specific device */
+	BTRFS_READ_POLICY_DEVID,
 #endif
 	BTRFS_NR_READ_POLICY,
 };
@@ -442,6 +444,9 @@ struct btrfs_fs_devices {
 	/* Min contiguous reads before switching to next device. */
 	int min_contiguous_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
-- 
2.46.1