[PATCH RFC 6/6] btrfs: implement shutdown_bdev super operation callback

[Qu Wenruo <wqu@xxxxxxxx>]

For this callback, btrfs will:

- Go degraded if the fs can still maintain RW operations

- Shutdown if the fs can not maintain RW operations

I know the shutdown can be a little overkilled, if one has a RAID1
metadata and RAID0 data, in that case one can still read data with 50%
rate to got some good data.

But it can also be as bad as the only device went missing for a single
device btrfs.

So here we go safe other than sorry when handling missing device.

Signed-off-by: Qu Wenruo <wqu@xxxxxxxx>
---
 fs/btrfs/super.c   | 34 ++++++++++++++++++++++++++++++++++
 fs/btrfs/volumes.c |  2 ++
 fs/btrfs/volumes.h |  5 +++++
 3 files changed, 41 insertions(+)

diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index 82ce0625b2f0..f82f9be41321 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -2397,6 +2397,39 @@ static long btrfs_free_cached_objects(struct super_block *sb, struct shrink_cont
 	return 0;
 }
 
+static void btrfs_shutdown_bdev(struct super_block *sb, struct block_device *bdev)
+{
+	struct btrfs_fs_info *fs_info = btrfs_sb(sb);
+	struct btrfs_device *device;
+	struct btrfs_dev_lookup_args lookup_args = { .devt = bdev->bd_dev };
+
+	mutex_lock(&fs_info->fs_devices->device_list_mutex);
+	device = btrfs_find_device(fs_info->fs_devices, &lookup_args);
+	mutex_unlock(&fs_info->fs_devices->device_list_mutex);
+	if (!device) {
+		btrfs_warn(fs_info, "unable to find btrfs device for block device '%pg'",
+			   bdev);
+		return;
+	}
+	set_bit(BTRFS_DEV_STATE_MISSING, &device->dev_state);
+	device->fs_devices->missing_devices++;
+	if (test_and_clear_bit(BTRFS_DEV_STATE_WRITEABLE, &device->dev_state)) {
+		list_del_init(&device->dev_alloc_list);
+		device->fs_devices->rw_devices--;
+	}
+	if (!btrfs_check_rw_degradable(fs_info, device)) {
+		btrfs_warn_in_rcu(fs_info,
+	"btrfs device id %llu path %s has gone missing, can not maintain read-write",
+				  device->devid, btrfs_dev_name(device));
+		btrfs_shutdown(fs_info);
+		return;
+	}
+	btrfs_warn_in_rcu(fs_info,
+	"btrfs device id %llu path %s has gone missing, continue degraded",
+			  device->devid, btrfs_dev_name(device));
+	btrfs_set_opt(fs_info->mount_opt, DEGRADED);
+}
+
 static const struct super_operations btrfs_super_ops = {
 	.drop_inode	= btrfs_drop_inode,
 	.evict_inode	= btrfs_evict_inode,
@@ -2412,6 +2445,7 @@ static const struct super_operations btrfs_super_ops = {
 	.unfreeze_fs	= btrfs_unfreeze,
 	.nr_cached_objects = btrfs_nr_cached_objects,
 	.free_cached_objects = btrfs_free_cached_objects,
+	.shutdown_bdev	= btrfs_shutdown_bdev,
 };
 
 static const struct file_operations btrfs_ctl_fops = {
diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 541d17ca5dcf..5639df71ef91 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -6792,6 +6792,8 @@ static bool dev_args_match_fs_devices(const struct btrfs_dev_lookup_args *args,
 static bool dev_args_match_device(const struct btrfs_dev_lookup_args *args,
 				  const struct btrfs_device *device)
 {
+	if (args->devt)
+		return device->devt == args->devt;
 	if (args->missing) {
 		if (test_bit(BTRFS_DEV_STATE_IN_FS_METADATA, &device->dev_state) &&
 		    !device->bdev)
diff --git a/fs/btrfs/volumes.h b/fs/btrfs/volumes.h
index afa71d315c46..2c5e3ebc5a2b 100644
--- a/fs/btrfs/volumes.h
+++ b/fs/btrfs/volumes.h
@@ -652,6 +652,11 @@ struct btrfs_dev_lookup_args {
 	u64 devid;
 	u8 *uuid;
 	u8 *fsid;
+	/*
+	 * If devt is specified, all other members will be ignored as it is
+	 * enough to uniquely locate a device.
+	 */
+	dev_t devt;
 	bool missing;
 };
 
-- 
2.49.0