I throw my kids backpacks in the washing machine all the time. I'm always amazined when I see filthy lunchbags and backpacks at school, that other people don't do this. I hang it to dry, and have never had a smidge of a problem. If it's really grungy, spray it with a stain remover first.

We washing machine wash canvas suitcases as well.

I have washed both backpacks and canvas lunch bags. In the summer I hang them outside to dry. In the winter I put them on the heating vent!
I make sure that I have my washing machine set on high water level. I have a top loader - no experience with a front loader.

I've washed backpacks, duffel bags, etc. plenty of times. I've had no problems.

SIL washes those huge heavy canvas LL Bean totes. They loose some of that stiff "stand up alone" feeling but come out fine. She uses a top loader too. Sandy

backpacks should be ok, but my boys have the LL Bean rolling bags and you can't wash those.