Plus you usually don't want empty pages sitting around doing nothing; it's a waste of memory. All "free" pages can instead be used as swap/filesystem cache. If a memory request comes in, then the page is yanked from the page cache, wiped, and given straight to whoever needs it. If the original contents (before it was yanked) are needed by the pagecache, it's merely faulted back in, pulling the data from its backing store, into a new location (and doesn't need pre-wiping for this purpose). Yes there is a case for fresh memory upon system boot, but as that only occures once per page, it's really not worth bogging the kernel down with code for.