linux-kernel - Re: Use of memmap= to forcibly recover memory in 3GB-4GB range

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Date:	Tue, 15 Jan 2013 18:28:49 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Alex Villacís Lasso <a_villacis@...osanto.com>
CC:	linux-kernel@...r.kernel.org
Subject: Re: Use of memmap= to forcibly recover memory in 3GB-4GB range -
 is this safe?

On 01/15/2013 05:47 PM, Alex Villacís Lasso wrote:
>
> The system boots, apparently normally, and I can see the additional
> "memory" in all system reports. However, I cannot quite shake the
> feeling that this "memory" might be in fact an illusion, and an attempt
> to use it will wrap around to the bottom of the memory and corrupt
> anything there. Or worse.
>
> Some tests that I have tried:
> 1) I have tried to occupy as much memory as possible, by starting two
> virtual machines, plus one instance of eclipse, a browser, and a
> bittorrent client, while running the graphical desktop. I have seen the
> free memory (as reported by "top") fall to under 200 Mb with no apparent
> instability, so this should prove that the extra memory is real, right?
> 2) I have recompiled the kernel to support the memtest parameter. When
> using it, the extra memory segment appears to be as healthy as other
> areas of memory. However this might only mean that it is wrapping into
> healthy low RAM.
>
> Is my reasoning sane? Is there a way to know, once and for all, whether
> the extra "memory" is real and safe to use or not?

Maybe you can get memtest86+ to test this phantom memory?  But yes, it 
does sound like a BIOS bug.

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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