Fix DirectIO cli toggle behaviour.

src/cli.rs

@@ -31,18 +31,23 @@ pub struct Args {
     #[arg(short, long, default_value_t = crate::FB_SECTOR_SIZE)]
     pub sector_size: usize,
 
-    // !!! ArgAction behaviour is backwards !!!
-    // ArgAction::SetFalse by default evaluates to true,
-    // ArgAction::SetTrue by default evaluates to false.
+    // !!! ArgAction behaviour is backwards to what you want to think !!!
+    // ArgAction::SetFalse sets default value to true,
+    // ArgAction::SetTrue sets default value to false.
+    //
+    // This is because ArgAction refers to the action that should happen *when* the flag is
+    // provided.
+    // I.e. a flag to disable something should take action to set the value as false.
     //
     /// Upon encountering a read error, reopen the source file before continuing.
     #[arg(short, long, action = ArgAction::SetTrue)]
     pub reopen_on_error: bool,
 
     /// Use O_DIRECT to bypass kernel buffer when reading.
+    /// It is very unlikely that you will want to disable this.
     //
     // BSD seems to support O_DIRECT, but MacOS for certain does not.
     #[cfg(all(unix, not(target_os = "macos")))]
-    #[arg(short, long = "direct", action = ArgAction::SetFalse)]
+    #[arg(short = 'd', long = "no-direct", action = ArgAction::SetFalse)]
     pub direct_io: bool,
 }