--- linux-2.6.23/arch/um/drivers/ubd_kern.c	2007-12-04 14:49:28.000000000 -0800
+++ new/arch/um/drivers/ubd_kern.c	2007-12-04 14:55:32.000000000 -0800
@@ -1069,6 +1069,12 @@
 	io_req->length = len;
 	io_req->error = 0;
 	io_req->sector_mask = 0;
+/* At startup, the maximum .length is 64K, and blk_queue_max_sectors()
+ * has not been called yet.  So do_io() could require 64K/(1<<9) = 128 bits:
+ * 32 in sector_mask, 64 in cow_offset, 32 in bitmap_words[0].
+ * 2007-12-04 jreiser (valgrind/memcheck)
+ */
+	io_req->bitmap_words[0] = 0;
 
 	io_req->op = (rq_data_dir(req) == READ) ? UBD_READ : UBD_WRITE;
 	io_req->offsets[0] = 0;