When you do not use the SWD and SDA pins for debugging (having set them to GPIO) they act just like "normal" IO pins. You may (and may not) configure them having internal pullups and other drive modes as shown in the pin's datasheet. When set to GPIO you still can use the pins for programming the device using a programmer.
Hi Noritomo san,
Is it OK to keep the SWD pin open?
Are you asking in cases when SWD is not being used? In that case they act like regular IO pins. . Normally it suggested to keep the unused pins in HighZ mode( which is by default).
Does the SWD pin have an internal pull-up like the XRES pin?
The SWDIO have internal pull-up and SWDCK will be have an internal pull-down normally when SWD is enabled.