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

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

> On Mon, 2009-06-15 at 20:42 +0200, Ingo Molnar wrote:
> > * 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 ...
>
> How will you manage the max depth?

if (++depth == MAX_DEPTH) {
print_all_entries_and_nasty_warning();
/* hope we'll live long enough for the syslog to touch disk */
depth = 0;
}
unbalanced kmap is a bad bug - the easier we make it to catch, the
better. The system wouldnt survive anyway.

Ingo


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