dm thin: fix pool_io_hints to avoid looking at max_hw_sectors

Simplify the pool_io_hints code that works to establish a max_sectors
value that is a power-of-2 factor of the thin-pool's blocksize.  The
biggest associated improvement is that the DM thin-pool is no longer
concerning itself with the data device's max_hw_sectors when adjusting
max_sectors.

This fixes the relative fragility of the original "dm thin: adjust
max_sectors_kb based on thinp blocksize" commit that only became
apparent when testing was performed using a DM thin-pool ontop of a
virtio_blk device.  One proposed upstream patch detailed the problems
inherent in virtio_blk: https://lkml.org/lkml/2014/11/20/611

So even though virtio_blk incorrectly set its max_hw_sectors it actually
helped make it clear that we need DM thinp to be tolerant of any future
Linux driver that incorrectly sets max_hw_sectors.

We only need to be concerned with modifying the thin-pool device's
max_sectors limit if it is smaller than the thin-pool's blocksize.  In
this case the value of max_sectors does become a limiting factor when
upper layers (e.g. filesystems) construct their bios.  But if the
hardware can support IOs larger than the thin-pool's blocksize the user
is encouraged to adjust the thin-pool's data device's max_sectors
accordingly -- doing so will enable the thin-pool to inherit the
established user-defined max_sectors.
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-thin.c

@@ -3587,27 +3587,20 @@ static void pool_io_hints(struct dm_target *ti, struct queue_limits *limits)
 	sector_t io_opt_sectors = limits->io_opt >> SECTOR_SHIFT;
 
 	/*
-	 * Adjust max_sectors_kb to highest possible power-of-2
-	 * factor of pool->sectors_per_block.
+	 * If max_sectors is smaller than pool->sectors_per_block adjust it
+	 * to the highest possible power-of-2 factor of pool->sectors_per_block.
+	 * This is especially beneficial when the pool's data device is a RAID
+	 * device that has a full stripe width that matches pool->sectors_per_block
+	 * -- because even though partial RAID stripe-sized IOs will be issued to a
+	 *    single RAID stripe; when aggregated they will end on a full RAID stripe
+	 *    boundary.. which avoids additional partial RAID stripe writes cascading
 	 */
-	if (limits->max_hw_sectors & (limits->max_hw_sectors - 1))
-		limits->max_sectors = rounddown_pow_of_two(limits->max_hw_sectors);
-	else
-		limits->max_sectors = limits->max_hw_sectors;
-
 	if (limits->max_sectors < pool->sectors_per_block) {
 		while (!is_factor(pool->sectors_per_block, limits->max_sectors)) {
 			if ((limits->max_sectors & (limits->max_sectors - 1)) == 0)
 				limits->max_sectors--;
 			limits->max_sectors = rounddown_pow_of_two(limits->max_sectors);
 		}
-	} else if (block_size_is_power_of_two(pool)) {
-		/* max_sectors_kb is >= power-of-2 thinp blocksize */
-		while (!is_factor(limits->max_sectors, pool->sectors_per_block)) {
-			if ((limits->max_sectors & (limits->max_sectors - 1)) == 0)
-				limits->max_sectors--;
-			limits->max_sectors = rounddown_pow_of_two(limits->max_sectors);
-		}
 	}
 
 	/*