dm cache: fix stacking of geometry limits

Do not blindly override the queue limits (specifically io_min and
io_opt).  Allow traditional stacking of these limits if io_opt is a
factor of the cache's data block size.

Without this patch mkfs.xfs does not recognize the cache device's
provided limits as a useful geometry (e.g. raid) so these hints are
ignored.  This was due to setting io_min to a useless value.
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>
Acked-by: Joe Thornber <ejt@redhat.com>

drivers/md/dm-cache-target.c

@@ -2612,9 +2612,17 @@ static void set_discard_limits(struct cache *cache, struct queue_limits *limits)
 static void cache_io_hints(struct dm_target *ti, struct queue_limits *limits)
 {
 	struct cache *cache = ti->private;
+	uint64_t io_opt_sectors = limits->io_opt >> SECTOR_SHIFT;
 
+	/*
+	 * If the system-determined stacked limits are compatible with the
+	 * cache's blocksize (io_opt is a factor) do not override them.
+	 */
+	if (io_opt_sectors < cache->sectors_per_block ||
+	    do_div(io_opt_sectors, cache->sectors_per_block)) {
 		blk_limits_io_min(limits, 0);
 		blk_limits_io_opt(limits, cache->sectors_per_block << SECTOR_SHIFT);
+	}
 	set_discard_limits(cache, limits);
 }