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

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Message-ID: <527BEA91.6050603@linux.intel.com>
Date:	Thu, 07 Nov 2013 11:31:29 -0800
From:	"H. Peter Anvin" <hpa@...ux.intel.com>
To:	Olof Johansson <olof@...om.net>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: Corrupted low memory in v3.9+

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.

	-hpa


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