Btrfs: allow block group cache writeout outside critical section in commit

We loop through all of the dirty block groups during commit and write
the free space cache.  In order to make sure the cache is currect, we do
this while no other writers are allowed in the commit.

If a large number of block groups are dirty, this can introduce long
stalls during the final stages of the commit, which can block new procs
trying to change the filesystem.

This commit changes the block group cache writeout to take appropriate
locks and allow it to run earlier in the commit.  We'll still have to
redo some of the block groups, but it means we can get most of the work
out of the way without blocking the entire FS.

Signed-off-by: Chris Mason <clm@fb.com>

1 files changed, 8 insertions, 0 deletions

diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index 1df0d9d..83051fa 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -1491,6 +1491,12 @@ struct btrfs_fs_info {
 	struct mutex chunk_mutex;
 	struct mutex volume_mutex;
 
+	/*
+	 * this is taken to make sure we don't set block groups ro after
+	 * the free space cache has been allocated on them
+	 */
+	struct mutex ro_block_group_mutex;
+
 	/* this is used during read/modify/write to make sure
 	 * no two ios are trying to mod the same stripe at the same
 	 * time
@@ -3407,6 +3413,8 @@ int btrfs_inc_extent_ref(struct btrfs_trans_handle *trans,
 			 u64 bytenr, u64 num_bytes, u64 parent,
 			 u64 root_objectid, u64 owner, u64 offset, int no_quota);
 
+int btrfs_start_dirty_block_groups(struct btrfs_trans_handle *trans,
+				   struct btrfs_root *root);
 int btrfs_write_dirty_block_groups(struct btrfs_trans_handle *trans,
 				    struct btrfs_root *root);
 int btrfs_setup_space_cache(struct btrfs_trans_handle *trans,