Re: [PATCH v2] btrfs: prevent subvol with swapfile from being deleted

[David Sterba <dsterba@suse.cz>]

On Wed, Mar 23, 2022 at 03:10:32PM +0800, Kaiwen Hu wrote:
> 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.
> 
> Reviewed-by: Robbie Ko <robbieko@synology.com>
> Signed-off-by: Kaiwen Hu <kevinhu@synology.com>

Added to misc-next, thanks.

> --- 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");

I've added the subvolume id to the two messages, otherwise it's not a
very useful for user.