btrfs-progs: make device discard process interruptible

The ioctl for the whole range is not interruptible, which can be
annoying when the discard is not wanted but user forgets to use the -K
option.

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

utils.c

@@ -52,8 +52,10 @@
 #define BLKDISCARD	_IO(0x12,119)
 #endif
 
-static int
-discard_blocks(int fd, u64 start, u64 len)
+/*
+ * Discard the given range in one go
+ */
+static int discard_range(int fd, u64 start, u64 len)
 {
 	u64 range[2] = { start, len };
 
@@ -62,6 +64,26 @@ discard_blocks(int fd, u64 start, u64 len)
 	return 0;
 }
 
+/*
+ * Discard blocks in the given range in 1G chunks, the process is interruptible
+ */
+static int discard_blocks(int fd, u64 start, u64 len)
+{
+	while (len > 0) {
+		/* 1G granularity */
+		u64 chunk_size = min_t(u64, len, 1*1024*1024*1024);
+		int ret;
+
+		ret = discard_range(fd, start, chunk_size);
+		if (ret)
+			return ret;
+		len -= chunk_size;
+		start += chunk_size;
+	}
+
+	return 0;
+}
+
 static u64 reference_root_table[] = {
 	[1] =	BTRFS_ROOT_TREE_OBJECTID,
 	[2] =	BTRFS_EXTENT_TREE_OBJECTID,