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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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