Re: [rfc] no ZERO_PAGE?

David Miller
2007-04-04T20:11:11Z

> Anyway, I'm not against this, but I can see somebody actually *wanting*
> the ZERO page in some cases. I've used the fact for TLB testing, for
> example, by just doing a big malloc(), and knowing that the kernel will
> re-use the ZERO_PAGE so that I don't get any cache effects (well, at least
> not any *physical* cache effects. Virtually indexed cached will still show
> effects of it, of course, but I haven't cared).
>
> That's an example of an app that actually cares about the page allocation
> (or, in this case, the lack there-of). Not an important one, but maybe
> there are important ones that care?

If we're going to consider this seriously, there is a case I know of.
Look at flush_dcache_page()'s test for ZERO_PAGE() on sparc64, there
is an instructive comment:

/* Do not bother with the expensive D-cache flush if it
* is merely the zero page. The 'bigcore' testcase in GDB
* causes this case to run millions of times.
*/
if (page == ZERO_PAGE(0))
return;

basically what the GDB test case does it mmap() an enormous anonymous
area, not touch it, then dump core.

As I understand the patch being considered to remove ZERO_PAGE(), this
kind of core dump will cause a lot of pages to be allocated, probably
eating up a lot of system time as well as memory.