FAILED: patch "[PATCH] btrfs: prevent subvol with swapfile from being deleted" failed to apply to 5.4-stable tree

The patch below does not apply to the 5.4-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 60021bd754c6ca0addc6817994f20290a321d8d6 Mon Sep 17 00:00:00 2001
From: Kaiwen Hu <[email protected]>
Date: Wed, 23 Mar 2022 15:10:32 +0800
Subject: [PATCH] btrfs: prevent subvol with swapfile from being deleted

A subvolume with an active swapfile must not be deleted otherwise it would not be possible to deactivate it.

After the subvolume is deleted, we cannot swapoff the swapfile in this deleted subvolume because the path is unreachable. The swapfile is still active and holding references, the filesystem cannot be unmounted.

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.

index b976f757571f..5aab6af88349 100644
@@ -4487,6 +4487,13 @@ 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 %llu with active swapfile",
+			   root->root_key.objectid);
+		return -EPERM;
+	}
 	root_flags = btrfs_root_flags(&dest->root_item);
 	btrfs_set_root_flags(&dest->root_item,
 			     root_flags | BTRFS_ROOT_SUBVOL_DEAD);

@@ -11110,8 +11117,23 @@ static int btrfs_swap_activate(struct swap_info_struct *sis, struct file *file,
 	 * set. We use this counter to prevent snapshots. We must increment it
 	 * before walking the extents because we don't want a concurrent
 	 * snapshot to run after we've already checked the extents.
+	 *
+	 * It is possible that subvolume is marked for deletion but still not
+	 * removed yet. To prevent this race, we check the root status before
+	 * activating the swapfile.
 	 */
+	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 %llu is being deleted",
+			root->root_key.objectid);
+		return -EPERM;
+	}
 	atomic_inc(&root->nr_swapfiles);
+	spin_unlock(&root->root_item_lock);
 
 	isize = ALIGN_DOWN(inode->i_size, fs_info->sectorsize);