Re: x86_64/HOTPLUG_CPU: NULL dereference doesn't #PF with init_level4_pgt

[Zwane Mwaikambo <zwane@arm.linux.org.uk>]

On Thu, 1 Dec 2005, Andi Kleen wrote:

> On Wed, Nov 30, 2005 at 08:03:33PM -0800, Zwane Mwaikambo wrote:
> > NULL dereferences don't cause a page fault if the 4th level pagetable 
> > being used is init_level4_pgt because we never zap_low_mappings. Since 
> > the idle thread uses init_level4_pgt any bad dereferences happening there 
> > (e.g. from interrupts) won't cause a fault. Andi would you be fine with 
> > switching the idle threads to a different level4?
> 
> That recently changed. Are you sure it's still the case?
> 
> idle threads should always run with lazy TLB, no different mms.
> That's important for performance.
> 
> If a NULL reference causes a oops or not depends on if user space
> from the last process mapped a page to NULL or not.

Ah thanks Andi, yes NULL reference causes an oops in the current -git 
repository, i hadn't seen that change so i had last tested it on 2.6.13. 
Sorry for the noise.

	Zwane