linux-kernel - Re: [rfc] no ZERO

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Date:	Wed, 04 Apr 2007 13:11:11 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	torvalds@...ux-foundation.org
Cc:	npiggin@...e.de, hugh@...itas.com, akpm@...ux-foundation.org,
	linux-mm@...ck.org, tee@....com, holt@....com, andrea@...e.de,
	linux-kernel@...r.kernel.org
Subject: Re: [rfc] no ZERO_PAGE?

From: Linus Torvalds <torvalds@...ux-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.
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