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>

fs/btrfs/ctree.h

@@ -559,6 +559,9 @@ enum {
 
 	/* Indicate that the discard workqueue can service discards. */
 	BTRFS_FS_DISCARD_RUNNING,
+
+	/* Indicate that we need to cleanup space cache v1 */
+	BTRFS_FS_CLEANUP_SPACE_CACHE_V1,
 };
 
 /*

fs/btrfs/disk-io.c

@@ -2888,6 +2888,7 @@ void btrfs_clear_oneshot_options(struct btrfs_fs_info *fs_info)
 int btrfs_start_pre_rw_mount(struct btrfs_fs_info *fs_info)
 {
 	int ret;
+	const bool cache_opt = btrfs_test_opt(fs_info, SPACE_CACHE);
 	bool clear_free_space_tree = false;
 
 	if (btrfs_test_opt(fs_info, CLEAR_CACHE) &&
@@ -2940,6 +2941,12 @@ int btrfs_start_pre_rw_mount(struct btrfs_fs_info *fs_info)
 		}
 	}
 
+	if (cache_opt != btrfs_free_space_cache_v1_active(fs_info)) {
+		ret = btrfs_set_free_space_cache_v1_active(fs_info, cache_opt);
+		if (ret)
+			goto out;
+	}
+
 	ret = btrfs_resume_balance_async(fs_info);
 	if (ret)
 		goto out;
@@ -3410,6 +3417,7 @@ int __cold open_ctree(struct super_block *sb, struct btrfs_fs_devices *fs_device
 			return ret;
 		}
 	}
+
 	set_bit(BTRFS_FS_OPEN, &fs_info->flags);
 
 clear_oneshot:

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

fs/btrfs/free-space-cache.h

@@ -135,6 +135,8 @@ int btrfs_trim_block_group_bitmaps(struct btrfs_block_group *block_group,
 				   u64 *trimmed, u64 start, u64 end, u64 minlen,
 				   u64 maxlen, bool async);
 
+bool btrfs_free_space_cache_v1_active(struct btrfs_fs_info *fs_info);
+int btrfs_set_free_space_cache_v1_active(struct btrfs_fs_info *fs_info, bool active);
 /* Support functions for running our sanity tests */
 #ifdef CONFIG_BTRFS_FS_RUN_SANITY_TESTS
 int test_add_free_space_entry(struct btrfs_block_group *cache,

fs/btrfs/super.c

@@ -552,7 +552,6 @@ int btrfs_parse_options(struct btrfs_fs_info *info, char *options,
 {
 	substring_t args[MAX_OPT_ARGS];
 	char *p, *num;
-	u64 cache_gen;
 	int intarg;
 	int ret = 0;
 	char *compress_type;
@@ -562,10 +561,9 @@ int btrfs_parse_options(struct btrfs_fs_info *info, char *options,
 	bool saved_compress_force;
 	int no_compress = 0;
 
-	cache_gen = btrfs_super_cache_generation(info->super_copy);
 	if (btrfs_fs_compat_ro(info, FREE_SPACE_TREE))
 		btrfs_set_opt(info->mount_opt, FREE_SPACE_TREE);
-	else if (cache_gen) {
+	else if (btrfs_free_space_cache_v1_active(info)) {
 		if (btrfs_is_zoned(info)) {
 			btrfs_info(info,
 			"zoned: clearing existing space cache");
@@ -1864,6 +1862,8 @@ static inline void btrfs_remount_begin(struct btrfs_fs_info *fs_info,
 static inline void btrfs_remount_cleanup(struct btrfs_fs_info *fs_info,
 					 unsigned long old_opts)
 {
+	const bool cache_opt = btrfs_test_opt(fs_info, SPACE_CACHE);
+
 	/*
 	 * We need to cleanup all defragable inodes if the autodefragment is
 	 * close or the filesystem is read only.
@@ -1880,6 +1880,10 @@ static inline void btrfs_remount_cleanup(struct btrfs_fs_info *fs_info,
 	else if (btrfs_raw_test_opt(old_opts, DISCARD_ASYNC) &&
 		 !btrfs_test_opt(fs_info, DISCARD_ASYNC))
 		btrfs_discard_cleanup(fs_info);
+
+	/* If we toggled space cache */
+	if (cache_opt != btrfs_free_space_cache_v1_active(fs_info))
+		btrfs_set_free_space_cache_v1_active(fs_info, cache_opt);
 }
 
 static int btrfs_remount(struct super_block *sb, int *flags, char *data)

fs/btrfs/transaction.c

@@ -1796,6 +1796,8 @@ static void update_super_roots(struct btrfs_fs_info *fs_info)
 	super->root_level = root_item->level;
 	if (btrfs_test_opt(fs_info, SPACE_CACHE))
 		super->cache_generation = root_item->generation;
+	else if (test_bit(BTRFS_FS_CLEANUP_SPACE_CACHE_V1, &fs_info->flags))
+		super->cache_generation = 0;
 	if (test_bit(BTRFS_FS_UPDATE_UUID_TREE_GEN, &fs_info->flags))
 		super->uuid_tree_generation = root_item->generation;
 }