loop: regularize upgrading the block size for direct I/O

The LOOP_CONFIGURE path automatically upgrades the block size to that
of the underlying file for O_DIRECT file descriptors, but the
LOOP_SET_BLOCK_SIZE path does not.  Fix this by lifting the code to
pick the block size into common code.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Link: https://lore.kernel.org/r/20240617060532.127975-7-hch@lst.deSigned-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/loop.c

@@ -975,10 +975,24 @@ loop_set_status_from_info(struct loop_device *lo,
 	return 0;
 }
 
+static unsigned short loop_default_blocksize(struct loop_device *lo,
+		struct block_device *backing_bdev)
+{
+	/* In case of direct I/O, match underlying block size */
+	if ((lo->lo_backing_file->f_flags & O_DIRECT) && backing_bdev)
+		return bdev_logical_block_size(backing_bdev);
+	return SECTOR_SIZE;
+}
+
 static int loop_reconfigure_limits(struct loop_device *lo, unsigned short bsize)
 {
+	struct file *file = lo->lo_backing_file;
+	struct inode *inode = file->f_mapping->host;
 	struct queue_limits lim;
 
+	if (!bsize)
+		bsize = loop_default_blocksize(lo, inode->i_sb->s_bdev);
+
 	lim = queue_limits_start_update(lo->lo_queue);
 	lim.logical_block_size = bsize;
 	lim.physical_block_size = bsize;
@@ -997,7 +1011,6 @@ static int loop_configure(struct loop_device *lo, blk_mode_t mode,
 	int error;
 	loff_t size;
 	bool partscan;
-	unsigned short bsize;
 	bool is_loop;
 
 	if (!file)
@@ -1076,15 +1089,7 @@ static int loop_configure(struct loop_device *lo, blk_mode_t mode,
 	if (!(lo->lo_flags & LO_FLAGS_READ_ONLY) && file->f_op->fsync)
 		blk_queue_write_cache(lo->lo_queue, true, false);
 
-	if (config->block_size)
-		bsize = config->block_size;
-	else if ((lo->lo_backing_file->f_flags & O_DIRECT) && inode->i_sb->s_bdev)
-		/* In case of direct I/O, match underlying block size */
-		bsize = bdev_logical_block_size(inode->i_sb->s_bdev);
-	else
-		bsize = 512;
-
-	error = loop_reconfigure_limits(lo, bsize);
+	error = loop_reconfigure_limits(lo, config->block_size);
 	if (WARN_ON_ONCE(error))
 		goto out_unlock;