linux-kernel - Re: [rfc] no ZERO

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Date:	Thu, 5 Apr 2007 04:03:47 +0200
From:	Nick Piggin <npiggin@...e.de>
To:	David Miller <davem@...emloft.net>
Cc:	torvalds@...ux-foundation.org, 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?

On Wed, Apr 04, 2007 at 01:11:11PM -0700, David Miller wrote:
> 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.

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.
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