mm: why we should clear page when do anonymous page fault

handle_pte_fault()
do_anonymous_page()
alloc_zeroed_user_highpage_movable()

We will alloc a zeroed page when do anonymous page fault, I don't know why should clear it? just for safe?

If user space program do like the following, there are two memset 0, right? kernel alloc zeroed page, and user memset 0 it again, this will waste a lot of time.

main()

{
	...
	vaddr = malloc(size)
	if (vaddr)
		memset(vaddr, 0, size);
	...
}

Thanks,
Xishi Qiu

I believe this is mainly for security reason.

To zero a highmem page, we could avoid another process peeking into the page that is (highly likely) just released by another process, who might well have put its confidential
data in that very page.

IIRC, Windows zeros the pages at freeing time. Linux instead does it lazily.

And for the userspace zeroing action, it is another problem - user just wants a clean, definitive
context to act on ( and we can be sure he/she is a self-disciplined guy who does not peek into
other's secret, but we can not assume that for all).

Thanks,
Jianyu Zhan

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to [email protected]. For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"[email protected]"> [email protected] </a>