Re: [patch] mm: fix PageUptodate memory ordering bug

[Andi Kleen <ak@suse.de>]

On Wednesday 02 January 2008 00:26:34 Nick Piggin wrote:
> On Sun, Dec 30, 2007 at 05:33:15PM +0100, Ingo Molnar wrote:
> > 
> > * Nick Piggin <npiggin@suse.de> wrote:
> > 
> > > > Sounds worthwhile, if we can't do it via altinstructions.
> > > 
> > > Altinstructions means we still have code bloat, and sometimes extra 
> > > branches etc (an extra 900 bytes of icache in mm/ alone, even before 
> > > my fix). I'll let Linus or one of the x86 guys weigh in, though. It's 
> > > a really sad cost for distro kernels to carry.
> > 
> > hm, we should at minimum display a warning if the workaround is not 
> > enabled and such a kernel is booted on a true PPro that is affected by 
> > this.
> 
> The patch does have the warning:
>   printk(KERN_INFO "Pentium Pro with Errata#66, #92 detected. Limiting maxcpus to 1.
>          Enable CONFIG_X86_BROKEN_PPRO_SMP to run with multiple CPUs\n");

Haven't seen the full patch, but the printk suggest you're changing the max_cpus 
variable. That is not 100% safe because user space could hot plug CPUs later
using sysfs. The only safe way would be to limit cpu_possible_map

-Andi

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