Why is clear_user_highpage necessary in do_anonymous_page()?

[Long Pu]

  I wonder why clear_user_highpage is necessary if we don't concern security.
When we write  to a anonymous page the very first time, we don't care
whatever the original contents of that page is. And
clear_user_highpage() takes a lot of cpu time under certain workload
such as compiling kernel.

 Is there any reason I don't know to reserve it?
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