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

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Date:	Tue, 12 Jun 2007 14:38:28 -0700
From:	"Ray Lee" <ray-lk@...rabbit.org>
To:	"Pavel Machek" <pavel@....cz>
Cc:	"Jesse Barnes" <jesse.barnes@...el.com>,
	"Andi Kleen" <andi@...stfloor.org>, linux-kernel@...r.kernel.org,
	"Justin Piszcz" <jpiszcz@...idpixels.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>
Subject: Re: [PATCH] trim memory not covered by WB MTRRs

On 6/12/07, Pavel Machek <pavel@....cz> 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).
> > > >
> > > > + if ((highest_addr >> PAGE_SHIFT) != end_pfn) {
> > > > +         printk(KERN_WARNING "***************\n");
> > > > +         printk(KERN_WARNING "**** WARNING: likely BIOS bug\n");
> > > > +         printk(KERN_WARNING "**** MTRRs don't cover all of "
> > > > +                "memory, trimmed %ld pages\n", end_pfn -
> > > > +                (highest_addr >> PAGE_SHIFT));
> > > > +         printk(KERN_WARNING "***************\n");
> > > > +         end_pfn = highest_addr >> PAGE_SHIFT;
> > >
> > > Missing 4K of memory is not worth 4K of junk in syslog per boot. Can
> > > you drop the stars and stop shouting?
> >
> > How missing about 1G of memory?  We already discussed this, and Andi and
> > Venki felt that either a panic or a really obnoxious message was the
> > way to go...
>
> Just use panic, then.
>                                                                         Pavel,
>         who still thinks anyone missing 1GB of ram will not miss
>         friendly notice in dmesg, even if it goes without 20 stars.

Panicking when it's not necessary is anti-social. If the kernel can
continue, then it should, unless it's a correctness issue that may
cause data corruption. Given that the kernel can even work around the
problem now, throwing a panic is even less warranted.

Ray
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