linux-kernel - Re: [rfc] no ZERO

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Date:	Wed, 4 Apr 2007 17:10:37 +0100 (BST)
From:	Hugh Dickins <hugh@...itas.com>
To:	Andrea Arcangeli <andrea@...e.de>
cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Nick Piggin <npiggin@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Memory Management List <linux-mm@...ck.org>,
	tee@....com, holt@....com,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [rfc] no ZERO_PAGE?

On Wed, 4 Apr 2007, Andrea Arcangeli wrote:
> On Wed, Apr 04, 2007 at 08:35:30AM -0700, Linus Torvalds wrote:
> > 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).
> 
> Ok, those cases wanting the same zero page, could be fairly easily
> converted to an mmap over /dev/zero

No, MAP_SHARED mmap of /dev/zero uses shmem, which allocates distinct
pages for this (because in general tmpfs doesn't know if a readonly
file will be written to later on), and MAP_PRIVATE mmap of /dev/zero
uses the zeromap stuff which we were hoping to eliminate too
(though not in Nick's initial patch).

Looks like a job for /dev/same_page_over_and_over_again.

> (without having to run 4k large mmap syscalls or nonlinear).

You scared me, I made no sense of that at first: ah yes,
repeatedly mmap'ing the same page can be done those ways.

Hugh
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