btrfs-progs: check: batch v1 space cache inodes when clearing

Currently v1 space cache clearing will delete one cache inode just in
one transaction, and then start a new transaction to delete the next
inode.

This is far from efficient and can make the already slow v1 space cache
deleting even slower, as large fs has tons of cache inodes to delete.

This patch will speed up the process by batching up to 16 inode deletion
into one transaction.

A quick benchmark of deleting 702 v1 space cache inodes would look like
this:

Unpatched:		4.898s
Patched:		0.087s

Which is obviously a big win.

Reported-by: Joshua <joshua@mailmag.net>
Link: https://lore.kernel.org/linux-btrfs/0b4cf70fc883e28c97d893a3b2f81b11@mailmag.net/
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

check/main.c

@@ -9892,33 +9892,65 @@ out:
 	return bad_roots;
 }
 
+/*
+ * Number of free space cache inodes to delete in one transaction.
+ *
+ * This is to speedup the v1 space cache deletion for large fs.
+ */
+#define NR_BLOCK_GROUP_CLUSTER		(16)
+
 static int clear_free_space_cache(void)
 {
 	struct btrfs_trans_handle *trans;
 	struct btrfs_block_group *bg_cache;
+	int nr_handled = 0;
 	u64 current = 0;
 	int ret = 0;
 
+	trans = btrfs_start_transaction(gfs_info->tree_root, 0);
+	if (IS_ERR(trans)) {
+		ret = PTR_ERR(trans);
+		errno = -ret;
+		error("failed to start a transaction: %m");
+		return ret;
+	}
+
 	/* Clear all free space cache inodes and its extent data */
 	while (1) {
 		bg_cache = btrfs_lookup_first_block_group(gfs_info, current);
 		if (!bg_cache)
 			break;
-		ret = btrfs_clear_free_space_cache(gfs_info, bg_cache);
-		if (ret < 0)
+		ret = btrfs_clear_free_space_cache(trans, bg_cache);
+		if (ret < 0) {
+			btrfs_abort_transaction(trans, ret);
 			return ret;
+		}
+		nr_handled++;
+
+		if (nr_handled == NR_BLOCK_GROUP_CLUSTER) {
+			ret = btrfs_commit_transaction(trans, gfs_info->tree_root);
+			if (ret < 0) {
+				errno = -ret;
+				error("failed to start a transaction: %m");
+				return ret;
+			}
+			trans = btrfs_start_transaction(gfs_info->tree_root, 0);
+			if (IS_ERR(trans)) {
+				ret = PTR_ERR(trans);
+				errno = -ret;
+				error("failed to start a transaction: %m");
+				return ret;
+			}
+		}
 		current = bg_cache->start + bg_cache->length;
 	}
 
-	/* Don't forget to set cache_generation to -1 */
-	trans = btrfs_start_transaction(gfs_info->tree_root, 0);
-	if (IS_ERR(trans)) {
-		error("failed to update super block cache generation");
-		return PTR_ERR(trans);
-	}
 	btrfs_set_super_cache_generation(gfs_info->super_copy, (u64)-1);
-	btrfs_commit_transaction(trans, gfs_info->tree_root);
-
+	ret = btrfs_commit_transaction(trans, gfs_info->tree_root);
+	if (ret < 0) {
+		errno = -ret;
+		error("failed to start a transaction: %m");
+	}
 	return ret;
 }
 

kernel-shared/free-space-cache.c

@@ -895,10 +895,10 @@ next:
 	}
 }
 
-int btrfs_clear_free_space_cache(struct btrfs_fs_info *fs_info,
+int btrfs_clear_free_space_cache(struct btrfs_trans_handle *trans,
 				 struct btrfs_block_group *bg)
 {
-	struct btrfs_trans_handle *trans;
+	struct btrfs_fs_info *fs_info = trans->fs_info;
 	struct btrfs_root *tree_root = fs_info->tree_root;
 	struct btrfs_path path;
 	struct btrfs_key key;
@@ -909,10 +909,6 @@ int btrfs_clear_free_space_cache(struct btrfs_fs_info *fs_info,
 	int slot;
 	int ret;
 
-	trans = btrfs_start_transaction(tree_root, 1);
-	if (IS_ERR(trans))
-		return PTR_ERR(trans);
-
 	btrfs_init_path(&path);
 
 	key.objectid = BTRFS_FREE_SPACE_OBJECTID;
@@ -1016,7 +1012,5 @@ int btrfs_clear_free_space_cache(struct btrfs_fs_info *fs_info,
 	}
 out:
 	btrfs_release_path(&path);
-	if (!ret)
-		btrfs_commit_transaction(trans, tree_root);
 	return ret;
 }

kernel-shared/free-space-cache.h

@@ -57,6 +57,6 @@ void unlink_free_space(struct btrfs_free_space_ctl *ctl,
 		       struct btrfs_free_space *info);
 int btrfs_add_free_space(struct btrfs_free_space_ctl *ctl, u64 offset,
 			 u64 bytes);
-int btrfs_clear_free_space_cache(struct btrfs_fs_info *fs_info,
+int btrfs_clear_free_space_cache(struct btrfs_trans_handle *trans,
 				 struct btrfs_block_group *bg);
 #endif