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

[Nick Piggin <npiggin@suse.de>]

On Wed, Jan 02, 2008 at 10:01:27PM +0100, Andi Kleen wrote:
> 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

Hmm, but I did want to allow it to be overridden via maxcpus= command line (or
hotplug I guess, I hadn't thought of the hotplug case but it allows runtime
override).

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