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    SubjectRe: [PATCH 09/10] Change table chaining layout
    From: Linus Torvalds <>
    Date: Mon, 22 Oct 2007 14:47:38 -0700 (PDT)

    > On Mon, 22 Oct 2007, Alan Cox wrote:
    > > Still doesn't answer the rather more important question - why not just
    > > stick a NULL on the end instead of all the nutty hacks ?
    > You still do need one bit for the discontiguous case, so it's not like you
    > can avoid the hacks anyway (unless you just blow up the structure
    > entirely) and make it a separate member). So once you have that
    > bit+pointer, using a separate NULL entry isn't exactly prettier.
    > Especially as we actally want to see the difference between
    > "end-of-allocation" and "not yet filled in", so you shouldn't use NULL
    > anyway, you should probably use something like "all-ones".

    Indeed that's the crux of the matter, we need to express a trinary
    state "end of scatterlist, next entry is linear, next entry is
    indirect" plus a pointer for the indirect case.

    Generally, Jens was doing a good job cooking up the patch that
    implemented this fully and I took care of making sure tricky
    ports like sparc64 built cleanly etc.

    He went away for a few days, but when he gets back we should seriously
    work on integrating his work.

    I fully recognize Alan's m68k on-stack alignment concern. The on-stack
    cases are troublesome in other ways as well, and I think therefore the
    way to move forward is to convert those to some kind of dynamic scheme.
    Usually such code is working in a locked context on some object
    (crypto instance, for example) and thus the scatterlist chunk can
    be embedded into that object for ensured alignment.
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