linux-kernel - Re: [PATCH] trim memory not covered by WB MTRRs

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Date:	Thu, 7 Jun 2007 13:40:55 -0400 (EDT)
From:	Justin Piszcz <jpiszcz@...idpixels.com>
To:	Jesse Barnes <jesse.barnes@...el.com>
cc:	Andi Kleen <andi@...stfloor.org>, linux-kernel@...r.kernel.org,
	"Eric W. Biederman" <ebiederm@...ssion.com>
Subject: Re: [PATCH] trim memory not covered by WB MTRRs



On Thu, 7 Jun 2007, Jesse Barnes wrote:

> On Thursday, June 7, 2007 1:16 am Andi Kleen wrote:
>> On Wed, Jun 06, 2007 at 12:29:23PM -0700, Jesse Barnes wrote:
>>> On some machines, buggy BIOSes don't properly setup WB MTRRs to
>>> cover all available RAM, meaning the last few megs (or even gigs)
>>> of memory will be marked uncached.  Since Linux tends to allocate
>>> from high memory addresses first, this causes the machine to be
>>> unusably slow as soon as the kernel starts really using memory
>>> (i.e. right around init time).
>>
>> In theory -- while not recommended -- a BIOS could also
>> use a default fallback MTRR for cached and use explicit MTRRs to
>> map the non existing ranges uncached. Would it make sense to handle
>> this case?
>
> Probably.  I could just check the default memory type and bail out if
> it's cacheable.
>
>> Should also probably have some command line option
>> to disable the check in case something bad happens with it.
>
> Sure.
>
>> Another thing that might be sense to investigate in relationship
>> to this patch is large page mappings with MTRRs. iirc P4 and also K8
>> splits pages internally with MTRR boundaries and might have some
>> other bad side effects. Should we use this as hints to use 4K pages
>> for the boundary areas?
>
> Or I could trim to the nearest large page boundary...  We'd lose a
> little more memory but it would keep things simple.
>
> Jesse
>

How much more memory are we going to lose?  Is mem= a better option if its 
going to keep decreasing?
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