linux-kernel - Re: [PATCH] mm: fix page_mkclean

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Message-ID: <20061228101311.GA9672@flint.arm.linux.org.uk>
Date:	Thu, 28 Dec 2006 10:13:11 +0000
From:	Russell King <rmk+lkml@....linux.org.uk>
To:	Gordon Farquharson <gordonfarquharson@...il.com>
Cc:	Linus Torvalds <torvalds@...l.org>,
	David Miller <davem@...emloft.net>, ranma@...edrich.de,
	tbm@...ius.com, Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	andrei.popa@...eo.ro, Andrew Morton <akpm@...l.org>,
	hugh@...itas.com, nickpiggin@...oo.com.au, arjan@...radead.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: fix page_mkclean_one

On Wed, Dec 27, 2006 at 10:20:20PM -0700, Gordon Farquharson wrote:
> I have run the program a few times, and the output is pretty
> consistent. However, when I increase the target size, the difference
> between the expected and actual values is larger.
> 
> Written as (749)935(738)
> Chunk 1113 corrupted (1-1455)  (2965-323)
> Expected 89, got 93

This is not the corruption Linus is after.  Note that the corruption starts
at offset '1'.  Also note that:

89 = 1113 & 255
93 = 1113 & 255 | (1113 >> 8)

and if you look at glibc's memset() function, you'll notice that's exactly
what you expect if you pass a non-8bit value to it.  Ergo, what you're
seeing is utterly expected given glibc's memset() implementation on ARM.

Fixing Linus' test program to pass nr & 255 to memset results in clean
passes on 2.6.9 on TheCus N2100 (IOP8032x) and 2.6.16.9 StrongARM
machines (as would be expected.)

-- 
Russell King
 Linux kernel    2.6 ARM Linux   - http://www.arm.linux.org.uk/
 maintainer of:
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