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    SubjectRe: [PATCH 0/13] maps: pagemap, kpagemap, and related cleanups
    On Fri, Apr 13, 2007 at 08:51:42AM +0100, Christoph Hellwig wrote:
    > Umm, folks. systemtap basically means people compile kernel modules
    > from an odd scripting language with embedded C snipplets into kernel
    > modules. The kernel modules don't use normal exported APIs but use
    > kallsysms and dwarf info to poke into every possible private bit.
    > Saying you don't care the slightest whether oracle will load huge
    > amounts of code into the kernel that depends on intimate implementation
    > details, and that you don't even have source to to debug it is not what
    > I'd call "none of us need to care in the slightest", at least for those
    > of you working for distributions that may actually have to debug this
    > shit in the end.
    > While we're at it, what happened to the idea of tainting the kernel
    > as soon as krpobes are placed in the kernel to at least make people
    > aware of it?

    This is for a system monitoring app outside the database proper that
    just happens to be done by the same .com as makes the database. It's
    got little to do with the database itself apart from knowing how to
    tell the database to e.g. let fewer clients in. The part that deals
    with this is sort of like a custom procps that does things rather
    specifically how they need them, including being portable to other
    OS's IIRC, though the scope of the app is much larger than that.

    They're actually quite concerned about issues of this sort since they
    want to run all the time in the background in order to respond to
    system conditions, though probably not necessarily rapid-fire sorts of
    responses to second-by-second changes in conditions.

    Basically, they're not a debugging affair, and they need to be able to
    run in supported conditions. They're rather disinterested in things
    that would, say, taint the kernel or take customers out of supported
    configurations. They'll fall back to the known-grossly-inaccurate
    RSS-based estimates they're using now in preference to such.

    They don't want omnibus back doors into the kernel and I honestly
    expect them to NAK the systemtap solution. They really want the
    "uniquely attributable RSS" or "proportional RSS" reported directly,
    and it takes some doing to convince them that this can't be done
    directly for various reasons, e.g. floating point in the kernel won't
    fly. They can program sufficiently well to maintain a database of pfn's,
    pid's of processes mapping them, and user virtual addresses they're
    mapped at (easy enough to kick off a database instance for it if they
    don't feel comfortable just hashing the triples) and do the tabulation
    from there, though they're not happy having to do so much of the
    calculation themselves. Actually, I promised them reporting of mapcount
    which would make per-process UARSS/PRSS calculation able to be done on
    a process-by-process basis, though I can probably convince them to do
    whole-system pfn-by-pfn tabulation if such is lacking.

    -- wli
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