On both sides of the Atlantic, many of the people who are most upset about the new free trade deal between Canada and the European Union are dairy farmers. But they have opposite worries. The deal was nearly derailed by enraged farmers in the Wallonia region of Belgium because of how much they had been struggling. In Canada, by contrast, farmers are anxious because they have been doing so well. The way the country’s “supply management” system works now, Canadian dairy farms are almost guaranteed to prosper. Milk production is controlled by quotas, marketing boards keep prices high and stable, and import duties of up to 300 percent largely shut out competition from abroad. But after the deal, the Comprehensive Economic and Trade Agreement, which was signed on Sunday, comes into effect, much more imported cheese will be allowed to enter Canada duty-free from the Continent. And farmers worry that this one dent in their defenses could be the beginning of the end for supply management.