btrfs-progs: mkfs: support free space tree as -R option

Add a runtime feature (-R) flag for the free space tree. A filesystem
that is mkfs'd with -R free-space-tree then mounted with no options has
the same contents as one mkfs'd without the option, then mounted with
'-o space_cache=v2'.

The only tricky thing is in exactly how to call the tree creation code.
Using btrfs_create_free_space_tree as is did not quite work, because an
extra reference to the eb (root->commit_root) is leaked, which mkfs
complains about with a warning. I opted to follow how the uuid tree is
created by adding it to the dirty roots list for cleanup by
commit_tree_roots in commit_transaction. As a result,
btrfs_create_free_space_tree no longer exactly matches the version in
the kernel sources.

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

Documentation/mkfs.btrfs.asciidoc

@@ -257,6 +257,12 @@ permanent, this does not replace mount options.
 Enable quota support (qgroups). The qgroup accounting will be consistent,
 can be used together with '--rootdir'.  See also `btrfs-quota`(8).
 
+*free-space-tree*::
+(kernel support since 4.5)
++
+Enable the free space tree (mount option space_cache=v2) for persisting the
+free space cache.
+
 BLOCK GROUPS, CHUNKS, RAID
 --------------------------
 

common/fsfeatures.c

@@ -107,6 +107,12 @@ static const struct btrfs_feature runtime_features[] = {
 	{ "quota", BTRFS_RUNTIME_FEATURE_QUOTA, NULL,
 		VERSION_TO_STRING2(3, 4), NULL, 0, NULL, 0,
 		"quota support (qgroups)" },
+	{ "free-space-tree", BTRFS_RUNTIME_FEATURE_FREE_SPACE_TREE,
+		"free_space_tree",
+		VERSION_TO_STRING2(4, 5),
+		VERSION_TO_STRING2(4, 9),
+		NULL, 0,
+		"free space tree (space_cache=v2)" },
 	/* Keep this one last */
 	{ "list-all", BTRFS_FEATURE_LIST_ALL, NULL }
 };

common/fsfeatures.h

@@ -39,7 +39,8 @@
 
 #define BTRFS_FEATURE_LIST_ALL		(1ULL << 63)
 
-#define BTRFS_RUNTIME_FEATURE_QUOTA	(1ULL << 0)
+#define BTRFS_RUNTIME_FEATURE_QUOTA		(1ULL << 0)
+#define BTRFS_RUNTIME_FEATURE_FREE_SPACE_TREE	(1ULL << 1)
 
 void btrfs_list_all_fs_features(u64 mask_disallowed);
 void btrfs_list_all_runtime_features(u64 mask_disallowed);

kernel-shared/disk-io.c

@@ -1003,10 +1003,15 @@ int btrfs_setup_all_roots(struct btrfs_fs_info *fs_info, u64 root_tree_bytenr,
 		ret = find_and_setup_root(root, fs_info, BTRFS_FREE_SPACE_TREE_OBJECTID,
 					  fs_info->free_space_root);
 		if (ret) {
+			free(fs_info->free_space_root);
+			fs_info->free_space_root = NULL;
 			printk("Couldn't read free space tree\n");
 			return -EIO;
 		}
 		fs_info->free_space_root->track_dirty = 1;
+	} else {
+		free(fs_info->free_space_root);
+		fs_info->free_space_root = NULL;
 	}
 
 	ret = find_and_setup_log_root(root, fs_info, sb);

kernel-shared/free-space-tree.c

@@ -1433,6 +1433,7 @@ int btrfs_create_free_space_tree(struct btrfs_fs_info *fs_info)
 		goto abort;
 	}
 	fs_info->free_space_root = free_space_root;
+	add_root_to_dirty_list(free_space_root);
 
 	do {
 		block_group = btrfs_lookup_first_block_group(fs_info, start);

mkfs/main.c

@@ -34,6 +34,7 @@
 #include <blkid/blkid.h>
 #include "kernel-shared/ctree.h"
 #include "kernel-shared/disk-io.h"
+#include "kernel-shared/free-space-tree.h"
 #include "kernel-shared/volumes.h"
 #include "kernel-shared/transaction.h"
 #include "common/utils.h"
@@ -1495,6 +1496,13 @@ raid_groups:
 			goto out;
 		}
 	}
+	if (runtime_features & BTRFS_RUNTIME_FEATURE_FREE_SPACE_TREE) {
+		ret = btrfs_create_free_space_tree(fs_info);
+		if (ret < 0) {
+			error("failed to create free space tree: %d (%m)", ret);
+			goto out;
+		}
+	}
 	if (verbose) {
 		char features_buf[64];
 

tests/mkfs-tests/023-free-space-tree/test.sh

@@ -0,0 +1,17 @@
+#!/bin/bash
+# Basic check if mkfs supports the runtime feature free-space-tree
+
+source "$TEST_TOP/common"
+
+check_prereq mkfs.btrfs
+check_prereq btrfs
+
+setup_root_helper
+prepare_test_dev
+
+run_check_mkfs_test_dev -R free-space-tree
+
+run_check_stdout $SUDO_HELPER "$TOP/btrfs" inspect-internal dump-super "$TEST_DEV" | \
+	grep -q FREE_SPACE_TREE || _fail "free space tree not created"
+run_check_stdout $SUDO_HELPER "$TOP/btrfs" inspect-internal dump-super "$TEST_DEV" | \
+	grep -q FREE_SPACE_TREE_VALID || _fail "free space tree not valid"