[PATCH] Fix read-only superblock in case of subvol RO remount

[Goldwyn Rodrigues <rgoldwyn@suse.de>]

If a read-write root mount is remounted as read-only, the subvolume
is also set to read-only.

Use a rw_mounts counter to check the number of read-write mounts, and change
superblock to read-only only in case there are no read-write subvol mounts.

Since sb->s_flags can change while calling legacy_reconfigure(), use
sb->s_flags instead of fc->sb_flags to re-evaluate sb->s_flags in
reconfigure_super().

Test case:
dd if=/dev/zero of=btrfsfs seek=240 count=0 bs=1M
mkfs.btrfs btrfsfs
mount btrfsfs /mnt
btrfs subvol create /mnt/sv
mount -o remount,ro /mnt
mount -osubvol=/sv btrfsfs /mnt/sv
findmnt # /mnt is RO, /mnt/sv RW
mount -o remount,ro /mnt
findmnt # /mnt is RO, /mnt/sv RO as well
umount /mnt{/sv,}
rm btrfsfs

Reported-by: Fabian Vogt <fvogt@suse.com>
Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>

diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index a2991971c6b5..2bb6869f15af 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -1060,6 +1060,9 @@ struct btrfs_fs_info {
 	spinlock_t zone_active_bgs_lock;
 	struct list_head zone_active_bgs;
 
+	/* Count of subvol mounts read-write */
+	int rw_mounts;
+
 #ifdef CONFIG_BTRFS_FS_REF_VERIFY
 	spinlock_t ref_verify_lock;
 	struct rb_root block_tree;
diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index 33cfc9e27451..32941e11e551 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -1835,6 +1835,11 @@ static struct dentry *btrfs_mount(struct file_system_type *fs_type, int flags,
 	/* mount_subvol() will free subvol_name and mnt_root */
 	root = mount_subvol(subvol_name, subvol_objectid, mnt_root);
 
+	if (!IS_ERR(root) && !(flags & SB_RDONLY)) {
+		struct btrfs_fs_info *fs_info = btrfs_sb(mnt_root->mnt_sb);
+		fs_info->rw_mounts++;
+	}
+
 out:
 	return root;
 }
@@ -1958,6 +1963,9 @@ static int btrfs_remount(struct super_block *sb, int *flags, char *data)
 		goto out;
 
 	if (*flags & SB_RDONLY) {
+
+		if (--fs_info->rw_mounts > 0)
+			goto out;
 		/*
 		 * this also happens on 'umount -rf' or on shutdown, when
 		 * the filesystem is busy.
@@ -2057,6 +2065,8 @@ static int btrfs_remount(struct super_block *sb, int *flags, char *data)
 		if (ret)
 			goto restore;
 
+		fs_info->rw_mounts++;
+
 		btrfs_clear_sb_rdonly(sb);
 
 		set_bit(BTRFS_FS_OPEN, &fs_info->flags);
diff --git a/fs/super.c b/fs/super.c
index f1d4a193602d..fd528f76f14b 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -913,7 +913,8 @@ int reconfigure_super(struct fs_context *fc)
 	}
 
 	WRITE_ONCE(sb->s_flags, ((sb->s_flags & ~fc->sb_flags_mask) |
-				 (fc->sb_flags & fc->sb_flags_mask)));
+				 (sb->s_flags & fc->sb_flags_mask)));
+
 	/* Needs to be ordered wrt mnt_is_readonly() */
 	smp_wmb();
 	sb->s_readonly_remount = 0;

-- 
Goldwyn