btrfs: remove device item and update super block in the same transaction

[BUG]
There is a report that a btrfs has a bad super block num devices.

This makes btrfs to reject the fs completely.

  BTRFS error (device sdd3): super_num_devices 3 mismatch with num_devices 2 found here
  BTRFS error (device sdd3): failed to read chunk tree: -22
  BTRFS error (device sdd3): open_ctree failed

[CAUSE]
During btrfs device removal, chunk tree and super block num devs are
updated in two different transactions:

  btrfs_rm_device()
  |- btrfs_rm_dev_item(device)
  |  |- trans = btrfs_start_transaction()
  |  |  Now we got transaction X
  |  |
  |  |- btrfs_del_item()
  |  |  Now device item is removed from chunk tree
  |  |
  |  |- btrfs_commit_transaction()
  |     Transaction X got committed, super num devs untouched,
  |     but device item removed from chunk tree.
  |     (AKA, super num devs is already incorrect)
  |
  |- cur_devices->num_devices--;
  |- cur_devices->total_devices--;
  |- btrfs_set_super_num_devices()
     All those operations are not in transaction X, thus it will
     only be written back to disk in next transaction.

So after the transaction X in btrfs_rm_dev_item() committed, but before
transaction X+1 (which can be minutes away), a power loss happen, then
we got the super num mismatch.

[FIX]
Instead of starting and committing a transaction inside
btrfs_rm_dev_item(), start a transaction in side btrfs_rm_device() and
pass it to btrfs_rm_dev_item().

And only commit the transaction after everything is done.
Reported-by: Luca Béla Palkovics <luca.bela.palkovics@gmail.com>
Link: https://lore.kernel.org/linux-btrfs/CA+8xDSpvdm_U0QLBAnrH=zqDq_cWCOH5TiV46CKmp3igr44okQ@mail.gmail.com/
CC: stable@vger.kernel.org # 4.14+
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/volumes.c

@@ -1896,23 +1896,18 @@ static void update_dev_time(const char *device_path)
 	path_put(&path);
 }
 
-static int btrfs_rm_dev_item(struct btrfs_device *device)
+static int btrfs_rm_dev_item(struct btrfs_trans_handle *trans,
+			     struct btrfs_device *device)
 {
 	struct btrfs_root *root = device->fs_info->chunk_root;
 	int ret;
 	struct btrfs_path *path;
 	struct btrfs_key key;
-	struct btrfs_trans_handle *trans;
 
 	path = btrfs_alloc_path();
 	if (!path)
 		return -ENOMEM;
 
-	trans = btrfs_start_transaction(root, 0);
-	if (IS_ERR(trans)) {
-		btrfs_free_path(path);
-		return PTR_ERR(trans);
-	}
 	key.objectid = BTRFS_DEV_ITEMS_OBJECTID;
 	key.type = BTRFS_DEV_ITEM_KEY;
 	key.offset = device->devid;
@@ -1923,21 +1918,12 @@ static int btrfs_rm_dev_item(struct btrfs_device *device)
 	if (ret) {
 		if (ret > 0)
 			ret = -ENOENT;
-		btrfs_abort_transaction(trans, ret);
-		btrfs_end_transaction(trans);
 		goto out;
 	}
 
 	ret = btrfs_del_item(trans, root, path);
-	if (ret) {
-		btrfs_abort_transaction(trans, ret);
-		btrfs_end_transaction(trans);
-	}
-
 out:
 	btrfs_free_path(path);
-	if (!ret)
-		ret = btrfs_commit_transaction(trans);
 	return ret;
 }
 
@@ -2078,6 +2064,7 @@ int btrfs_rm_device(struct btrfs_fs_info *fs_info,
 		    struct btrfs_dev_lookup_args *args,
 		    struct block_device **bdev, fmode_t *mode)
 {
+	struct btrfs_trans_handle *trans;
 	struct btrfs_device *device;
 	struct btrfs_fs_devices *cur_devices;
 	struct btrfs_fs_devices *fs_devices = fs_info->fs_devices;
@@ -2098,7 +2085,7 @@ int btrfs_rm_device(struct btrfs_fs_info *fs_info,
 
 	ret = btrfs_check_raid_min_devices(fs_info, num_devices - 1);
 	if (ret)
-		goto out;
+		return ret;
 
 	device = btrfs_find_device(fs_info->fs_devices, args);
 	if (!device) {
@@ -2106,27 +2093,22 @@ int btrfs_rm_device(struct btrfs_fs_info *fs_info,
 			ret = BTRFS_ERROR_DEV_MISSING_NOT_FOUND;
 		else
 			ret = -ENOENT;
-		goto out;
+		return ret;
 	}
 
 	if (btrfs_pinned_by_swapfile(fs_info, device)) {
 		btrfs_warn_in_rcu(fs_info,
 		  "cannot remove device %s (devid %llu) due to active swapfile",
 				  rcu_str_deref(device->name), device->devid);
-		ret = -ETXTBSY;
-		goto out;
+		return -ETXTBSY;
 	}
 
-	if (test_bit(BTRFS_DEV_STATE_REPLACE_TGT, &device->dev_state)) {
-		ret = BTRFS_ERROR_DEV_TGT_REPLACE;
-		goto out;
-	}
+	if (test_bit(BTRFS_DEV_STATE_REPLACE_TGT, &device->dev_state))
+		return BTRFS_ERROR_DEV_TGT_REPLACE;
 
 	if (test_bit(BTRFS_DEV_STATE_WRITEABLE, &device->dev_state) &&
-	    fs_info->fs_devices->rw_devices == 1) {
-		ret = BTRFS_ERROR_DEV_ONLY_WRITABLE;
-		goto out;
-	}
+	    fs_info->fs_devices->rw_devices == 1)
+		return BTRFS_ERROR_DEV_ONLY_WRITABLE;
 
 	if (test_bit(BTRFS_DEV_STATE_WRITEABLE, &device->dev_state)) {
 		mutex_lock(&fs_info->chunk_mutex);
@@ -2139,14 +2121,22 @@ int btrfs_rm_device(struct btrfs_fs_info *fs_info,
 	if (ret)
 		goto error_undo;
 
-	/*
-	 * TODO: the superblock still includes this device in its num_devices
-	 * counter although write_all_supers() is not locked out. This
-	 * could give a filesystem state which requires a degraded mount.
-	 */
-	ret = btrfs_rm_dev_item(device);
-	if (ret)
+	trans = btrfs_start_transaction(fs_info->chunk_root, 0);
+	if (IS_ERR(trans)) {
+		ret = PTR_ERR(trans);
 		goto error_undo;
+	}
+
+	ret = btrfs_rm_dev_item(trans, device);
+	if (ret) {
+		/* Any error in dev item removal is critical */
+		btrfs_crit(fs_info,
+			   "failed to remove device item for devid %llu: %d",
+			   device->devid, ret);
+		btrfs_abort_transaction(trans, ret);
+		btrfs_end_transaction(trans);
+		return ret;
+	}
 
 	clear_bit(BTRFS_DEV_STATE_IN_FS_METADATA, &device->dev_state);
 	btrfs_scrub_cancel_dev(device);
@@ -2229,7 +2219,8 @@ int btrfs_rm_device(struct btrfs_fs_info *fs_info,
 		free_fs_devices(cur_devices);
 	}
 
-out:
+	ret = btrfs_commit_transaction(trans);
+
 	return ret;
 
 error_undo:
@@ -2240,7 +2231,7 @@ int btrfs_rm_device(struct btrfs_fs_info *fs_info,
 		device->fs_devices->rw_devices++;
 		mutex_unlock(&fs_info->chunk_mutex);
 	}
-	goto out;
+	return ret;
 }
 
 void btrfs_rm_dev_replace_remove_srcdev(struct btrfs_device *srcdev)