linux-kernel - Re: Corrupted low memory in v3.9+

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Date:	Mon, 11 Nov 2013 12:35:03 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	"H. Peter Anvin" <hpa@...ux.intel.com>
Cc:	Olof Johansson <olof@...om.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Corrupted low memory in v3.9+


* H. Peter Anvin <hpa@...ux.intel.com> wrote:

> On 11/07/2013 11:02 AM, Olof Johansson wrote:
> >> [    0.000000] reserving inaccessible SNB gfx pages
> >> [    0.000000] memblock_reserve: [0x00000000000000-0x00000000100000]
> 
> This is on a Sandy Bridge system, which I guess I managed to miss the 
> first time.  Unfortunately low memory corruption is expected with SNB 
> graphics... this is why we unconditionally reserve all low memory on 
> SNB.
> 
> >> setup_arch+0xa2d/0xa41
> >> [...]
> >>
> >> Unfortunately x86 doesn't keep the memblock structures around, so 
> >> there's no way to verify after booting in debugfs, but based on the 
> >> above it should have been reserved properly.
> > 
> > *prod*
> > 
> > So, got a preference on solution for this? The warning seems harmless 
> > but still annoying to get used to ignoring false positives, etc.
> > 
> > Disable the low memory checker by default? Hide it behind a debug 
> > option (runtime or build time)?
> 
> I'm inclined to say disable it by default, but I'll let Ingo comment. 
> These days we default to reserving all of low memory other than the 
> trampoline (which we really can't avoid); leaving it in as a debug 
> option seems reasonable, but it is really questionable to me how much it 
> is useful to a general user.

If we reserve everything in low memory, all the time (which I very much 
argue we should do) then the checker becomes a no-op and can be removed.

Thanks,

	Ingo
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