Add more comments explaining how we block IPs

config/initializers/rack_attack_git_basic_auth.rb

@@ -1,4 +1,6 @@
 unless Rails.env.test?
+  # Tell the Rack::Attack Rack middleware to maintain an IP blacklist. We will
+  # update the blacklist from Grack::Auth#authenticate_user.
   Rack::Attack.blacklist('Git HTTP Basic Auth') do |req|
     Rack::Attack::Allow2Ban.filter(req.ip, Gitlab.config.rack_attack.git_basic_auth) do
       # This block only gets run if the IP was not already banned.

lib/gitlab/backend/grack_auth.rb

@@ -76,7 +76,10 @@ module Grack
       return user if user.present?
 
       # At this point, we know the credentials were wrong. We let Rack::Attack
-      # know there was a failed authentication attempt from this IP
+      # know there was a failed authentication attempt from this IP. This
+      # information is stored in the Rails cache (Redis) and will be used by
+      # the Rack::Attack middleware to decide whether to block requests from
+      # this IP.
       Rack::Attack::Allow2Ban.filter(@request.ip, Gitlab.config.rack_attack.git_basic_auth) do
         # Return true, so that Allow2Ban increments the counter (stored in
         # Rails.cache) for the IP