Re: [rfc] no ZERO_PAGE?

Nick Piggin
2007-04-05T02:03:47Z

On Wed, Apr 04, 2007 at 01:11:11PM -0700, David Miller wrote:
> From: Linus Torvalds <torvalds@linux-foundation.org>
> Date: Wed, 4 Apr 2007 08:35:30 -0700 (PDT)
>
> > 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.

Yeah. Well it is trivial to leave ZERO_PAGE in get_user_pages, however
in the longer run it would be nice to get rid of ZERO_PAGE completely
so we need an alternative.

I've been working on a patch for core dumping that can detect unfaulted
anonymous memory and skip it without doing the ZERO_PAGE comparision.