btrfs: extend balance filter limit to take minimum and maximum

The 'limit' filter is underdesigned, it should have been a range for
[min,max], with some relaxed semantics when one of the bounds is
missing. Besides that, using a full u64 for a single value is a waste of
bytes.

Let's fix both by extending the use of the u64 bytes for the [min,max]
range. This can be done in a backward compatible way, the range will be
interpreted only if the appropriate flag is set
(BTRFS_BALANCE_ARGS_LIMIT_RANGE).

Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Chris Mason <clm@fb.com>

1 files changed, 12 insertions, 2 deletions

diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index 3fa3c3b7bb66..460cd69b405e 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -846,8 +846,18 @@ struct btrfs_disk_balance_args {
         /* BTRFS_BALANCE_ARGS_* */
         __le64 flags;
 
-        /* BTRFS_BALANCE_ARGS_LIMIT value */
-        __le64 limit;
+        /*
+         * BTRFS_BALANCE_ARGS_LIMIT with value 'limit'
+         * BTRFS_BALANCE_ARGS_LIMIT_RANGE - the extend version can use minimum
+         * and maximum
+         */
+        union {
+                __le64 limit;
+                struct {
+                        __le32 limit_min;
+                        __le32 limit_max;
+                };
+        };
 
         __le64 unused[7];
 } __attribute__ ((__packed__));