[PATCH] UML - limit requests on COW devices to 32 sectors

[Jeff Dike]

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.

Signed-off-by: Jeff Dike <jdike@xxxxxxxxxxxxxxx>
--
 arch/um/drivers/ubd_kern.c |    2 ++
 1 file changed, 2 insertions(+)

Index: linux-2.6.17/arch/um/drivers/ubd_kern.c
===================================================================
--- linux-2.6.17.orig/arch/um/drivers/ubd_kern.c	2007-07-09 08:53:09.000000000 -0400
+++ linux-2.6.17/arch/um/drivers/ubd_kern.c	2007-07-09 18:08:33.000000000 -0400
@@ -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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