uml: limit request size on COWed devices

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@linux.intel.com>
Cc: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 2 insertions, 0 deletions

diff --git a/arch/um/drivers/ubd_kern.c b/arch/um/drivers/ubd_kern.c
index 0947f2e20045..fc27f6c72b41 100644
--- a/arch/um/drivers/ubd_kern.c
+++ b/arch/um/drivers/ubd_kern.c
@@ -712,6 +712,8 @@ static int ubd_add(int n, char **error_out)
         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";