[PATCH v6 08/10] btrfs: warn when remount will not change the free space tree

[Boris Burkov <boris@bur.io>]

If the remount is ro->ro, rw->ro, or rw->rw, we will not create or
clear the free space tree. This can be surprising, so print a warning
to dmesg to make the failure more visible. It is also important to
ensure that the space cache options (SPACE_CACHE, FREE_SPACE_TREE) are
consistent, so ensure those are set to properly match the current on
disk state (which won't be changing).

Signed-off-by: Boris Burkov <boris@bur.io>
---
 fs/btrfs/super.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index 527ab305ac68..ed5c80f92f78 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -1914,6 +1914,24 @@ static int btrfs_remount(struct super_block *sb, int *flags, char *data)
 	btrfs_resize_thread_pool(fs_info,
 		fs_info->thread_pool_size, old_thread_pool_size);
 
+	if (btrfs_test_opt(fs_info, FREE_SPACE_TREE) !=
+	    btrfs_fs_compat_ro(fs_info, FREE_SPACE_TREE) &&
+	    ((!sb_rdonly(sb) || *flags & SB_RDONLY))) {
+		btrfs_warn(fs_info,
+	   "remount supports changing free space tree only from ro to rw");
+		/*
+		 * Make sure free space cache options match the state on disk
+		 */
+		if (btrfs_fs_compat_ro(fs_info, FREE_SPACE_TREE)) {
+			btrfs_set_opt(fs_info->mount_opt, FREE_SPACE_TREE);
+			btrfs_clear_opt(fs_info->mount_opt, SPACE_CACHE);
+		}
+		if (btrfs_free_space_cache_v1_active(fs_info)) {
+			btrfs_clear_opt(fs_info->mount_opt, FREE_SPACE_TREE);
+			btrfs_set_opt(fs_info->mount_opt, SPACE_CACHE);
+		}
+	}
+
 	if ((bool)(*flags & SB_RDONLY) == sb_rdonly(sb))
 		goto out;
 
-- 
2.24.1