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

From: Ingo Molnar
Date: Mon Jun 15 2009 - 14:43:03 EST

* Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:

> On Mon, 2009-06-15 at 20:25 +0200, Ingo Molnar wrote:
> > * Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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);


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