We should do this.
<rdar://problem/6910754>
Created attachment 30552 [details] Proposed patch
Comment on attachment 30552 [details] Proposed patch r=me. A comment on why we prefer MADV_FREE over MADV_DONTNEED could be make the subtleties in this code a little less confusing.
Landed in r43988 with a comment explaining the change as per your suggestion.