Re: [tip:perfcounters/core] x86: Add NMI types for kmap_atomic

[Ingo Molnar <mingo@elte.hu>]

* Peter Zijlstra <peterz@infradead.org> wrote:

> On Mon, 2009-06-15 at 20:25 +0200, Ingo Molnar wrote:
> > * Peter Zijlstra <peterz@infradead.org> wrote:
> 
> > but ... look at the APIs i propose above. We dont need _any_ 
> > 'types'.
> > 
> > That type enumeration is basically an open-coded allocator. If we do 
> > a _real_ allocator (a balanced stack of atomic kmaps) we dont need 
> > any of those indices, and all the potential for mismatch goes away 
> > as well - a stack nests trivially with IRQ and NMI and arbitrary 
> > other contexts.
> 
> You want types because:
>  - they encode the intent, and can be verified
>  - they help keep track of the max nesting depth
> 
> In the proposed implementation all type code basically falls away 
> no ! CONFIG_DEBUG_VM, but is kept around for robustness.

But much of the fragility of the types (and their clumsiness - for 
example in highpte ops we have to know at which level of the 
pagetables we are, and use the right kind of index) is _precisely_ 
because we have the types ...

Unbalanced unmaps will be detected under CONFIG_DEBUG_VM: kunmap 
uses 'page' as a parameter which is checked against the pte entry - 
they must match.

I.e. it becomes a symmetric and expressive API:

	kaddr = kmap_atomic(page);
	...
	kunmap_atomic(page);

Hm?

	Ingo