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    SubjectRe: No more bits in vm_area_struct's vm_flags.
    On Tue, 29 Sep 2009, Christoph Lameter wrote:
    >
    > Another concern that has not been discussed is the increased cache
    > footprint due to a slightly enlarged vm data working set (there is also a
    > corresponding icache issue since additional accesses are needed).

    Using unsigned long long vm_flags makes no difference to cache footprint
    on 64-bit systems, being a no-op there; and I think these days, though
    we sure like our 32-bit systems to run well, we're not so anxious about
    saving every last cycle on them.

    >
    > Could we stick with the current size and do combinations of flags like we
    > do with page flags?

    Are we doing that? If you have some example like, when PG_slab is set
    then PG_owner_priv_1 means such-and-such, but if not not: okay, I'm
    fine with that.

    But if you're saying something like, if PG_reclaim is set at the same
    time as PG_buddy, then they mean the page is not a buddy or under
    reclaim, but brokenbacked: then I'm a bit (or even 32 bits) worried.

    > VM_HUGETLB cannot grow up and down f.e. and there are
    > certainly lots of other impossible combinations that can be used to put
    > more information into the flags.

    Where it makes sense, where it's understandable, okay: there may be a
    few which could naturally use combinations. But in general, no, I
    think we'd be asking for endless maintenance trouble if we change the
    meaning of some flags according to other flags.

    Hugh


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