[PATCH] btrfs: init device stats for seed devices

[Josef Bacik <josef@toxicpanda.com>]

We recently started recording device stats across the fleet, and noticed
a large increase in messages such as this

BTRFS warning (device dm-0): get dev_stats failed, not yet valid

on our tiers that use seed devices for their root devices.  This is
because we do not initialize the device stats for any seed devices if we
have a sprout device and mount using that sprout device.  The basic
steps for reproducing are

mkfs seed device
mount seed device
fill seed device
umount seed device
btrfstune -S 1 seed device
mount seed device
btrfs device add -f sprout device /mnt/wherever
umount /mnt/wherever
mount sprout device /mnt/wherever
btrfs device stats /mnt/wherever

This will fail with the above message in dmesg.

Fix this by iterating over the fs_devices->seed if they exist in
btrfs_init_dev_stats.  This fixed the problem and properly reports the
stats for both devices.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
---
 fs/btrfs/volumes.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index d7670e2a9f39..dab295880117 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -7225,7 +7225,7 @@ int btrfs_init_dev_stats(struct btrfs_fs_info *fs_info)
 	path = btrfs_alloc_path();
 	if (!path)
 		return -ENOMEM;
-
+again:
 	mutex_lock(&fs_devices->device_list_mutex);
 	list_for_each_entry(device, &fs_devices->devices, dev_list) {
 		int item_size;
@@ -7263,6 +7263,12 @@ int btrfs_init_dev_stats(struct btrfs_fs_info *fs_info)
 	}
 	mutex_unlock(&fs_devices->device_list_mutex);
 
+	/* If we have seed devices we need to init those stats as well. */
+	if (fs_devices->seed) {
+		fs_devices = fs_devices->seed;
+		goto again;
+	}
+
 	btrfs_free_path(path);
 	return ret < 0 ? ret : 0;
 }
-- 
2.24.1