Add a new option '-b' for btrfstune, to enable bg-tree feature for a
unmounted fs.

This feature will convert all BLOCK_GROUP_ITEMs in extent tree to bg
tree, by reusing the existing btrfs_convert_to_bg_tree() function.

Signed-off-by: Qu Wenruo <w...@suse.com>
---
 Documentation/btrfstune.asciidoc |  6 +++++
 btrfstune.c                      | 44 ++++++++++++++++++++++++++++++--
 2 files changed, 48 insertions(+), 2 deletions(-)

diff --git a/Documentation/btrfstune.asciidoc b/Documentation/btrfstune.asciidoc
index 1d6bc98deed8..ed54c2e1597f 100644
--- a/Documentation/btrfstune.asciidoc
+++ b/Documentation/btrfstune.asciidoc
@@ -26,6 +26,12 @@ means.  Please refer to the 'FILESYSTEM FEATURES' in 
`btrfs`(5).
 OPTIONS
 -------
 
+-b::
+(since kernel: 5.x)
++
+enable bg-tree feature (faster mount time for large fs), enabled by mkfs
+feature 'bg-tree'.
+
 -f::
 Allow dangerous changes, e.g. clear the seeding flag or change fsid. Make sure
 that you are aware of the dangers.
diff --git a/btrfstune.c b/btrfstune.c
index afa3aae35412..aa1ac568aef0 100644
--- a/btrfstune.c
+++ b/btrfstune.c
@@ -476,11 +476,39 @@ static void print_usage(void)
        printf("\t-m          change fsid in metadata_uuid to a random UUID\n");
        printf("\t            (incompat change, more lightweight than 
-u|-U)\n");
        printf("\t-M UUID     change fsid in metadata_uuid to UUID\n");
+       printf("\t-b          enable bg-tree feature (mkfs: bg-tree, for faster 
mount time)\n");
        printf("  general:\n");
        printf("\t-f          allow dangerous operations, make sure that you 
are aware of the dangers\n");
        printf("\t--help      print this help\n");
 }
 
+static int convert_to_bg_tree(struct btrfs_fs_info *fs_info)
+{
+       struct btrfs_trans_handle *trans;
+       int ret;
+
+       trans = btrfs_start_transaction(fs_info->tree_root, 1);
+       if (IS_ERR(trans)) {
+               ret = PTR_ERR(trans);
+               errno = -ret;
+               error("failed to start transaction: %m");
+               return ret;
+       }
+       ret = btrfs_convert_to_bg_tree(trans);
+       if (ret < 0) {
+               errno = -ret;
+               error("failed to convert: %m");
+               btrfs_abort_transaction(trans, ret);
+               return ret;
+       }
+       ret = btrfs_commit_transaction(trans, fs_info->tree_root);
+       if (ret < 0) {
+               errno = -ret;
+               error("failed to commit transaction: %m");
+       }
+       return ret;
+}
+
 int BOX_MAIN(btrfstune)(int argc, char *argv[])
 {
        struct btrfs_root *root;
@@ -491,6 +519,7 @@ int BOX_MAIN(btrfstune)(int argc, char *argv[])
        u64 seeding_value = 0;
        int random_fsid = 0;
        int change_metadata_uuid = 0;
+       bool to_bg_tree = false;
        char *new_fsid_str = NULL;
        int ret;
        u64 super_flags = 0;
@@ -501,7 +530,7 @@ int BOX_MAIN(btrfstune)(int argc, char *argv[])
                        { "help", no_argument, NULL, GETOPT_VAL_HELP},
                        { NULL, 0, NULL, 0 }
                };
-               int c = getopt_long(argc, argv, "S:rxfuU:nmM:", long_options, 
NULL);
+               int c = getopt_long(argc, argv, "S:rxfuU:nmM:b", long_options, 
NULL);
 
                if (c < 0)
                        break;
@@ -539,6 +568,9 @@ int BOX_MAIN(btrfstune)(int argc, char *argv[])
                        ctree_flags |= OPEN_CTREE_IGNORE_FSID_MISMATCH;
                        change_metadata_uuid = 1;
                        break;
+               case 'b':
+                       to_bg_tree = true;
+                       break;
                case GETOPT_VAL_HELP:
                default:
                        print_usage();
@@ -556,7 +588,7 @@ int BOX_MAIN(btrfstune)(int argc, char *argv[])
                return 1;
        }
        if (!super_flags && !seeding_flag && !(random_fsid || new_fsid_str) &&
-           !change_metadata_uuid) {
+           !change_metadata_uuid && !to_bg_tree) {
                error("at least one option should be specified");
                print_usage();
                return 1;
@@ -602,6 +634,14 @@ int BOX_MAIN(btrfstune)(int argc, char *argv[])
                return 1;
        }
 
+       if (to_bg_tree) {
+               ret = convert_to_bg_tree(root->fs_info);
+               if (ret < 0) {
+                       errno = -ret;
+                       error("failed to convert to bg-tree feature: %m");
+                       goto out;
+               }
+       }
        if (seeding_flag) {
                if (btrfs_fs_incompat(root->fs_info, METADATA_UUID)) {
                        fprintf(stderr, "SEED flag cannot be changed on a 
metadata-uuid changed fs\n");
-- 
2.23.0

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