UML - limit requests on COW devices to 32 sectors

COWed devices can't handle more than 32 (64 on x86_64) sectors in one request due to the size of the bitmap being carried around in the io_thread_req.

Enforce that by telling the block layer not to put too many sectors in requests to COWed devices.

--

 arch/um/drivers/ubd_kern.c |    2 ++
 1 file changed, 2 insertions(+)

Index: linux-2.6.17/arch/um/drivers/ubd_kern.c
===================================================================
@@ -712,6 +712,8 @@ static int ubd_add(int n, char **error_o
 	ubd_dev->queue->queuedata = ubd_dev;
 
 	blk_queue_max_hw_segments(ubd_dev->queue, MAX_SG);
+	if(ubd_dev->cow.file != NULL)
+		blk_queue_max_sectors(ubd_dev->queue, 8 * sizeof(long));
 	err = ubd_disk_register(MAJOR_NR, ubd_dev->size, n, &ubd_gendisk[n]);
 	if(err){
 		*error_out = "Failed to register device";

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