remove device mapper max_sectors restriction

The maximum size of a bio is restricted by DM to BLK_DEF_MAX_SECTORS although the I/O size can be increased for each member device above this value by using the /sys/block/queue/max_sectors interface.

IMHO the limitation when loading the dm table is unnecessary, the values for sub-devices are checked while set through the sysfs interface and the restrictions of the sub-devices are combined, the minimal values are used for the DM queue. Chopping this additionally to BLK_DEF_MAX_SECTORS makes it impossible to use IOs bigger than 512KB on big RAIDs accessed through multipath, even if the devices may support them. The trivial patch removes this limitation. Sub-devices' max_sectors need to be set before creating the DM device.

Patch is against 2.6.24. For older kernels max_hw_sectors needs to be set additionally.

---
 dm-table.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

@@ -901,10 +901,10 @@
 void dm_table_set_restrictions(struct dm_table *t, struct request_queue *q)
 {
 	/*
-	 * Make sure we obey the optimistic sub devices
+	 * Make sure we obey the minimum of the sub devices
 	 * restrictions.
 	 */
-	blk_queue_max_sectors(q, t->limits.max_sectors);
+	q->max_sectors = t->limits.max_sectors;
 	q->max_phys_segments = t->limits.max_phys_segments;
 	q->max_hw_segments = t->limits.max_hw_segments;
 	q->hardsect_size = t->limits.hardsect_size;