[PATCH] btrfs: prevent subvol with swapfile from being deleted

[Kaiwen Hu <kevinhu@synology.com>]

This patch prevent subvol being deleted when the subvol contains
any active swapfile.

Since the subvolume is deleted, we cannot swapoff the swapfile in
this deleted subvolume.  However, the swapfile is still active,
we unable to unmount this volume.  Let it into some deadlock
situation.

The test looks like this:
	mkfs.btrfs -f $dev > /dev/null
	mount $dev $mnt

	btrfs sub create $mnt/subvol
	touch $mnt/subvol/swapfile
	chmod 600 $mnt/subvol/swapfile
	chattr +C $mnt/subvol/swapfile
	dd if=/dev/zero of=$mnt/subvol/swapfile bs=1K count=4096
	mkswap $mnt/subvol/swapfile
	swapon $mnt/subvol/swapfile

	btrfs sub delete $mnt/subvol
	swapoff $mnt/subvol/swapfile  // failed: No such file or directory
	swapoff --all

	unmount $mnt  // target is busy.

To prevent above issue, we simply check that whether the subvolume
contains any active swapfile, and stop the deleting process.  This
behavior is like snapshot ioctl dealing with a swapfile.

Signed-off-by: Kaiwen Hu <kevinhu@synology.com>
---
 fs/btrfs/inode.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 5bbea5ec31fc..e388b9043710 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -4460,6 +4460,12 @@ int btrfs_delete_subvolume(struct inode *dir, struct dentry *dentry)
 			   dest->root_key.objectid);
 		return -EPERM;
 	}
+	if (atomic_read(&dest->nr_swapfiles)) {
+		spin_unlock(&dest->root_item_lock);
+		btrfs_warn(fs_info,
+			   "attempt to delete subvolume with active swapfile");
+		return -ETXTBSY;
+	}
 	root_flags = btrfs_root_flags(&dest->root_item);
 	btrfs_set_root_flags(&dest->root_item,
 			     root_flags | BTRFS_ROOT_SUBVOL_DEAD);
@@ -10419,7 +10425,17 @@ static int btrfs_swap_activate(struct swap_info_struct *sis, struct file *file,
 	 * before walking the extents because we don't want a concurrent
 	 * snapshot to run after we've already checked the extents.
 	 */
+	spin_lock(&root->root_item_lock);
+	if (btrfs_root_dead(root)) {
+		spin_unlock(&root->root_item_lock);
+		btrfs_exclop_finish(fs_info);
+		btrfs_warn(fs_info,
+	   "cannot activate swapfile because subvolume is marked for deletion");
+		return -EINVAL;
+	}
 	atomic_inc(&root->nr_swapfiles);
+	spin_unlock(&root->root_item_lock);
+
 
 	isize = ALIGN_DOWN(inode->i_size, fs_info->sectorsize);
 
-- 
2.25.1