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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 20:15 +0200, Ingo Molnar wrote:
    > > * Peter Zijlstra <peterz@infradead.org> wrote:
    > >
    > > > +static int kmap_type_to_context(enum km_type type)
    > > > +{
    > > > + switch (type) {
    > > > + case KM_BOUNCE_READ:
    > > > + return KM_CTX_USER;
    > > > + case KM_SKB_SUNRPC_DATA:
    > > > + return KM_CTX_USER;
    > > > + case KM_SKB_DATA_SOFTIRQ:
    > > > + return KM_CTX_SOFTIRQ;
    > > > + case KM_USER0:
    > > > + return KM_CTX_USER;
    > > > + case KM_USER1:
    > > > + return KM_CTX_USER;
    > > > + case KM_BIO_SRC_IRQ:
    > > > + return KM_CTX_IRQ;
    > > > + case KM_BIO_DST_IRQ:
    > > > + return KM_CTX_IRQ;
    > > > + case KM_PTE0:
    > > > + return KM_CTX_USER;
    > > > + case KM_PTE1:
    > > > + return KM_CTX_USER;
    > > > + case KM_IRQ0:
    > > > + return KM_CTX_IRQ;
    > > > + case KM_IRQ1:
    > > > + return KM_CTX_IRQ;
    > > > + case KM_SOFTIRQ0:
    > > > + return KM_CTX_SOFTIRQ;
    > > > + case KM_SOFTIRQ1:
    > > > + return KM_CTX_SOFTIRQ;
    > > > + case KM_NMI:
    > > > + return KM_CTX_NMI;
    > > > + case KM_NMI_PTE:
    > > > + return KM_CTX_NMI;
    > > > + }
    > > > +
    > > > + return KM_CTX_MAX;
    > >
    > > why not do a very simple stack of atomic kmaps, like Hugh suggested?
    > >
    > > That would mean a much simpler interface:
    > >
    > > kaddr = kmap_atomic(page);
    > >
    > > no index needed. The kunmap pops the entry off the stack:
    > >
    > > kunmap_atomic(kaddr);
    > >
    > > This becomes simpler too.
    > >
    > > Now, a stack can be overflown by imbalance - but that's easy to
    > > detect and existing entries are easily printed and thus the source
    > > of the leak is easily identified.
    > >
    > > In my book this design beats the current enumeration of kmap types
    > > indices hands down ... It would likely be much more robust as well,
    > > and much more easy to extend.
    > >
    > > Am i missing any subtlety?
    >
    > The above is mostly debug code used to validate the kmap_atomic
    > conditions.
    >
    > KM_CTX_NMI nests in KM_CTX_IRQ nests in KM_CTX_SOFTIRQ nests in
    > KM_CTX_USER.
    >
    > And validate that we indeed are in the context specified by the type.
    > That is, it will warn if we use KM_IRQ1 with KM_CTX_IRQ from user
    > context.
    >
    > Some of this was already captured in the old kmap debug code which I
    > removed.
    >
    > But yes, I should write that nicer..

    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.

    Ingo


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