Btrfs: create special free space cache inode

In order to save free space cache, we need an inode to hold the data, and we
need a special item to point at the right inode for the right block group.  So
first, create a special item that will point to the right inode, and the number
of extent entries we will have and the number of bitmaps we will have.  We
truncate and pre-allocate space everytime to make sure it's uptodate.

This feature will be turned on as soon as you mount with -o space_cache, however
it is safe to boot into old kernels, they will just generate the cache the old
fashion way.  When you boot back into a newer kernel we will notice that we
modified and not the cache and automatically discard the cache.

Signed-off-by: Josef Bacik <josef@redhat.com>

1 files changed, 6 insertions, 1 deletions

diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index 1776dbd8dc98..5c23eb8d6ba3 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -68,7 +68,7 @@ enum {
         Opt_nodatacow, Opt_max_inline, Opt_alloc_start, Opt_nobarrier, Opt_ssd,
         Opt_nossd, Opt_ssd_spread, Opt_thread_pool, Opt_noacl, Opt_compress,
         Opt_compress_force, Opt_notreelog, Opt_ratio, Opt_flushoncommit,
-        Opt_discard, Opt_err,
+        Opt_discard, Opt_space_cache, Opt_err,
 };
 
 static match_table_t tokens = {
@@ -92,6 +92,7 @@ static match_table_t tokens = {
         {Opt_flushoncommit, "flushoncommit"},
         {Opt_ratio, "metadata_ratio=%d"},
         {Opt_discard, "discard"},
+        {Opt_space_cache, "space_cache"},
         {Opt_err, NULL},
 };
 
@@ -235,6 +236,10 @@ int btrfs_parse_options(struct btrfs_root *root, char *options)
                 case Opt_discard:
                         btrfs_set_opt(info->mount_opt, DISCARD);
                         break;
+                case Opt_space_cache:
+                        printk(KERN_INFO "btrfs: enabling disk space caching\n");
+                        btrfs_set_opt(info->mount_opt, SPACE_CACHE);
+                        break;
                 case Opt_err:
                         printk(KERN_INFO "btrfs: unrecognized mount option "
                                "'%s'\n", p);