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    SubjectRe: [bug] ata subsystem related crash with latest -git


    On Wed, 17 Oct 2007, David Miller wrote:
    >
    > I believe that we have enough of a limited set of accessors to
    > sg->page that we can more aggressively encode things in the lower
    > bits.
    >
    > I'm thinking of encoding the low two bits of sg->page as
    > follows:
    >
    > 1) bits == 0
    >
    > then the SG list is linear and sg_next() is sg++
    >
    > 2) bits == 1
    >
    > the nest SG is an indirect chunk, sg_next() is
    > therefore something like:
    >
    > next = *((struct scatterlist **)(sg + 1));
    >
    > 3) bits == 2
    >
    > this is the last entry in the scatterlist, sg_next() is NULL
    >
    > So for the cases of ARCH_HAS_SG_CHAIN not being set (ie. back
    > compatible), we can do no bit encoding in page->flags and just do
    > sg_next() == sg++, as is done now.

    Yes, that sounds sane.

    Although I also wonder whether we want one global per-arch
    ARCH_HAS_SG_CHAIN, or perhaps have something more dynamic, which allows a
    per-SG-list choice (which in turn would require some kind of "head" entry
    that is passed into sg_next(), and in general descibes the SG list)

    Linus
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