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    SubjectRe: mmap dirty limits on 32 bit kernels (Was: [BUG] New Kernel Bugs)


    On Thu, 15 Nov 2007, Peter Zijlstra wrote:
    >
    > Something like this ought to do I guess. Although my
    > mapping_is_buffercache() is the ugliest thing. I'm sure that can be done
    > better.

    No, this absolutely sucks.

    Why?

    It's totally unacceptable to have per-mapping notions of how much memory
    we have. We used to do *exactly* that, and it's idiocy.

    The reason it's unacceptable idiocy is that it means that two processes
    that access different files will then have *TOTALLY*DIFFERENT* notions of
    what the "dirty limit" is. And as a result, one process will happily write
    lots and lots of dirty stuff and never throttle, and the other process
    will have to throttle all the time - and clean up after the process that
    didn't!

    See?

    The fact is, because we count dirty pages as one resource, we must also
    have *one* limit.

    So this patch is a huge regression. You might not notice it, because if
    everybody writes to the same kind of mapping, nobody will be hurt (they
    all have effectively the same global limit anyway), but you *will* notice
    if you ever have two different values of "highmem".

    Unacceptable. We used to do exactly what your patch does, and it got fixed
    once. We're not introducing that fundamentally broken concept again.

    Linus
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