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, 1 deletions

diff --git a/include/uapi/linux/btrfs.h b/include/uapi/linux/btrfs.h
index b6dec05c7196..11f13108b78b 100644
--- a/include/uapi/linux/btrfs.h
+++ b/include/uapi/linux/btrfs.h
@@ -217,7 +217,18 @@ struct btrfs_balance_args {
 
         __u64 flags;
 
-        __u64 limit;            /* limit number of processed chunks */
+        /*
+         * BTRFS_BALANCE_ARGS_LIMIT with value 'limit'
+         * BTRFS_BALANCE_ARGS_LIMIT_RANGE - the extend version can use minimum
+         * and maximum
+         */
+        union {
+                __u64 limit;            /* limit number of processed chunks */
+                struct {
+                        __u32 limit_min;
+                        __u32 limit_max;
+                };
+        };
         __u64 unused[7];
 } __attribute__ ((__packed__));