First - it's not plain canvas; it's Sunbella. According to the company's web site, to clean mildew, you mix a 1/4 of dishwashing liquid, 1 cup of bleach and 1 gallon of water.
https://www.sunbrella.cn/en-us/how-to-clean
We've used lemon juice in place of bleach; it's a lot easier to breathe while using to mix and works just as well.