blk-settings: round down io_opt to physical_block_size

There was a bug report [1] where the user got a warning alignment
inconsistency. The user has optimal I/O 16776704 (0xFFFE00) and physical
block size 4096. Note that the optimal I/O size may be set by the DMA
engines or SCSI controllers and they have no knowledge about the disks
attached to them, so the situation with optimal I/O not aligned to
physical block size may happen.

This commit makes blk_validate_limits round down optimal I/O size to the
physical block size of the block device.

Closes: https://lore.kernel.org/dm-devel/1426ad71-79b4-4062-b2bf-84278be66a5d@redhat.com/T/ [1]
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Fixes: a236346 ("block: take io_opt and io_min into account for max_sectors")
Cc: stable@vger.kernel.org	# v6.11+
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/3dc0014b-9690-dc38-81c9-4a316a2d4fb2@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-settings.c

@@ -248,6 +248,13 @@ int blk_validate_limits(struct queue_limits *lim)
 	if (lim->io_min < lim->physical_block_size)
 		lim->io_min = lim->physical_block_size;
 
+	/*
+	 * The optimal I/O size may not be aligned to physical block size
+	 * (because it may be limited by dma engines which have no clue about
+	 * block size of the disks attached to them), so we round it down here.
+	 */
+	lim->io_opt = round_down(lim->io_opt, lim->physical_block_size);
+
 	/*
 	 * max_hw_sectors has a somewhat weird default for historical reason,
 	 * but driver really should set their own instead of relying on this