linux-kernel - Re: Linux 2.6.20-rc7

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Date:	Thu, 01 Feb 2007 11:44:36 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	PaweÅ Sikora <pluto@...k.net>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Christoph Hellwig <hch@....de>,
	Pekka Enberg <penberg@...helsinki.fi>,
	"Siddha, Suresh B" <suresh.b.siddha@...el.com>,
	Martin Peschke <mp3@...ibm.com>, Andrew Morton <akpm@...l.org>,
	Adrian Bunk <bunk@...sta.de>
Subject: Re: Linux 2.6.20-rc7

Linus Torvalds wrote:

> 	if (free_pages <= min + z->lowmem_reserve[classzone_idx])
> 		return 0;
> 
> gets broken, because the negative 'free_pages' will look like a huge 
> unsigned positive number (and we'll make it unsigned becaue 'min' got 
> turned unsigned). There was a reason that thing was signed in the first 
> place, and neither me nor Andrew noticed.
> 
> Bad Nick. And bad me and Andrew for not noticing.

Sorry. I think I even wrote that comment at the top of the function.
And probably the function as well :(

> I should either revert that commit or just check for "free_pages" being 
> negative. The latter, in many ways, is probably better, because generally 
> we simply should never work with negative numbers in the kernel, so when 
> something potentially goes negative, we're probably just better off always 
> testing it explicitly anyway.
> 
> Nick, Andrew, any preferences? 

As Andrew says, it would need to be checked each time, because we have
nothing synchronising against free_pages at the top, or nr_free in the
loop.

We could make them both unsigned, and _add_ everything to min rather than
subtracting from free_pages?

-- 
SUSE Labs, Novell Inc.
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