btrfs: keep sb cache_generation consistent with space_cache

When mounting, btrfs uses the cache_generation in the super block to
determine if space cache v1 is in use. However, by mounting with
nospace_cache or space_cache=v2, it is possible to disable space cache
v1, which does not result in un-setting cache_generation back to 0.

In order to base some logic, like mount option printing in /proc/mounts,
on the current state of the space cache rather than just the values of
the mount option, keep the value of cache_generation consistent with the
status of space cache v1.

We ensure that cache_generation > 0 iff the file system is using
space_cache v1. This requires committing a transaction on any mount
which changes whether we are using v1. (v1->nospace_cache, v1->v2,
nospace_cache->v1, v2->v1).

Since the mechanism for writing out the cache generation is transaction
commit, but we want some finer grained control over when we un-set it,
we can't just rely on the SPACE_CACHE mount option, and introduce an
fs_info flag that mount can use when it wants to unset the generation.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Boris Burkov <boris@bur.io>
Signed-off-by: David Sterba <dsterba@suse.com>

1 files changed, 28 insertions, 0 deletions

diff --git a/fs/btrfs/free-space-cache.c b/fs/btrfs/free-space-cache.c
index fe4b2176a6e8..e71def319655 100644
--- a/fs/btrfs/free-space-cache.c
+++ b/fs/btrfs/free-space-cache.c
@@ -3763,6 +3763,34 @@ int btrfs_trim_block_group_bitmaps(struct btrfs_block_group *block_group,
 	return ret;
 }
 
+bool btrfs_free_space_cache_v1_active(struct btrfs_fs_info *fs_info)
+{
+	return btrfs_super_cache_generation(fs_info->super_copy);
+}
+
+int btrfs_set_free_space_cache_v1_active(struct btrfs_fs_info *fs_info, bool active)
+{
+	struct btrfs_trans_handle *trans;
+	int ret;
+
+	/*
+	 * update_super_roots will appropriately set
+	 * super_copy->cache_generation based on the SPACE_CACHE option, so all
+	 * we have to do is trigger a transaction commit.
+	 */
+	trans = btrfs_start_transaction(fs_info->tree_root, 0);
+	if (IS_ERR(trans))
+		return PTR_ERR(trans);
+
+	if (!active)
+		set_bit(BTRFS_FS_CLEANUP_SPACE_CACHE_V1, &fs_info->flags);
+
+	ret = btrfs_commit_transaction(trans);
+	clear_bit(BTRFS_FS_CLEANUP_SPACE_CACHE_V1, &fs_info->flags);
+
+	return ret;
+}
+
 #ifdef CONFIG_BTRFS_FS_RUN_SANITY_TESTS
 /*
  * Use this if you need to make a bitmap or extent entry specifically, it