linux-kernel - Re: [rfc] no ZERO

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Date:	Wed, 4 Apr 2007 08:35:30 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Nick Piggin <npiggin@...e.de>
cc:	Hugh Dickins <hugh@...itas.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Memory Management List <linux-mm@...ck.org>,
	tee@....com, holt@....com, Andrea Arcangeli <andrea@...e.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [rfc] no ZERO_PAGE?

On Wed, 4 Apr 2007, Nick Piggin wrote:
> 
> Shall I do a more complete patchset and ask Andrew to give it a
> run in -mm?

Do this trivial one first. See how it fares.

Although I don't know how much -mm will do for it. There is certainly not 
going to be any correctness problems, afaik, just *performance* problems. 
Does anybody do any performance testing on -mm?

That said, talking about correctness/performance problems:

> +	page_table = pte_offset_map_lock(mm, pmd, address, &ptl);
> +	if (likely(!pte_none(*page_table))) {
>  		inc_mm_counter(mm, anon_rss);
>  		lru_cache_add_active(page);
>  		page_add_new_anon_rmap(page, vma, address);

Isn't that test the wrong way around?

Shouldn't it be

	if (likely(pte_none(*page_table))) {

without any logical negation? Was this patch tested?

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?

			Linus
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