linux-kernel - Re: Testing RAM from userspace / question about memmap= arguments

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Date:	Wed, 26 Dec 2007 00:09:57 +0100
From:	Pavel Machek <pavel@....cz>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	Matthew Bloch <matthew@...emark.co.uk>,
	linux-kernel@...r.kernel.org
Subject: Re: Testing RAM from userspace / question about memmap= arguments

On Sat 2007-12-22 12:09:59, Arjan van de Ven wrote:
> On Tue, 18 Dec 2007 17:06:24 +0000
> Matthew Bloch <matthew@...emark.co.uk> wrote:
> 
> > Hi - I'm trying to come up with a way of thoroughly testing every byte
> > of RAM from within Linux on amd64 (so that it can be automated better
> > than using memtest86+), and came up with an idea which I'm not sure is
> > supported or practical.
> > 
> > The obvious problem with testing memory from user space is that you
> > can't mlock all of it, so the best you can do is about three quarters,
> > and hope that the rest of the memory is okay.
> 
> well... to be honest the more obvious problem will be that you won't be testing the RAM, you'll be testing the CPU's cache.. over and over again.
> 
> memtest86+ does various magic to basically bypass the caches (by disabling them ;-)...
> Doing that in a live kernel situation, and from userspace to boot...... that's... and issue.

Are you sure? I always assumed that memtest just used patterns bigger
than L1/L2 caches... ... and IIRC my celeron testing confirmed it, if
I disabled L2 cache in BIOS, memtest behave differently.

Anyway, if you can do iopl(), we may as well let you disable caches,
but you are right, that will need a kernel patch.
								Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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