[PATCH v7 07/12] btrfs: keep sb cache_generation consistent with space_cache

[Boris Burkov <boris@bur.io>]

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.

Signed-off-by: Boris Burkov <boris@bur.io>
---
 fs/btrfs/ctree.h            |  3 +++
 fs/btrfs/disk-io.c          |  8 ++++++++
 fs/btrfs/free-space-cache.c | 28 ++++++++++++++++++++++++++++
 fs/btrfs/free-space-cache.h |  3 +++
 fs/btrfs/super.c            | 10 +++++++---
 fs/btrfs/transaction.c      |  2 ++
 6 files changed, 51 insertions(+), 3 deletions(-)

diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index cb90a870b235..318303f53529 100644
--- a/fs/btrfs/ctree.h
+++ b/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,
 };
 
 /*
diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 64e5707f008b..fea467c421e7 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -2902,6 +2902,7 @@ void btrfs_clear_oneshot_options(struct btrfs_fs_info *fs_info)
 int btrfs_mount_rw(struct btrfs_fs_info *fs_info)
 {
 	int ret;
+	bool cache_opt = btrfs_test_opt(fs_info, SPACE_CACHE);
 	bool clear_free_space_tree = false;
 
 	if (btrfs_test_opt(fs_info, CLEAR_CACHE) &&
@@ -2955,6 +2956,12 @@ int btrfs_mount_rw(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;
@@ -3418,6 +3425,7 @@ 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:
diff --git a/fs/btrfs/free-space-cache.c b/fs/btrfs/free-space-cache.c
index 572c75d2169b..48f7bd050909 100644
--- a/fs/btrfs/free-space-cache.c
+++ b/fs/btrfs/free-space-cache.c
@@ -3971,6 +3971,34 @@ int btrfs_write_out_ino_cache(struct btrfs_root *root,
 	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
+	 * fs_info->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
diff --git a/fs/btrfs/free-space-cache.h b/fs/btrfs/free-space-cache.h
index bf8d127d2407..8f4bc5781cd3 100644
--- a/fs/btrfs/free-space-cache.h
+++ b/fs/btrfs/free-space-cache.h
@@ -149,6 +149,9 @@ 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,
diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index cca00cc0c98c..ff19f900cee1 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -547,7 +547,6 @@ 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;
@@ -557,10 +556,9 @@ 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))
 		btrfs_set_opt(info->mount_opt, SPACE_CACHE);
 
 	/*
@@ -1857,6 +1855,8 @@ 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)
 {
+	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.
@@ -1873,6 +1873,10 @@ 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)
diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
index 0e4063651047..43253b9996c3 100644
--- a/fs/btrfs/transaction.c
+++ b/fs/btrfs/transaction.c
@@ -1801,6 +1801,8 @@ 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;
 }
-- 
2.24.1