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    SubjectRe: RFC: COW for hugepages
    On Wed, Apr 07, 2004 at 07:01:26PM -0700, Andrew Morton wrote:
    > Benjamin Herrenschmidt <benh@kernel.crashing.org> wrote:
    > >
    > >
    > > > Implementing this for ppc64 only is just wrong. Before you do this
    > > > I would suggest to factor out the common code in the various hugetlbpage
    > > > implementations and then implement it in common code.
    > >
    > > Have you actually looked at it and how huge pages are implemented
    > > on the various architectures ?
    > >
    > > Honestly, I don't think we have any common abstraction on things
    > > like hugepte's etc... actually, archs aren't even required to use
    > > PTEs at all.
    > >
    > > I don't see how we can make that code arch-neutral, at least not
    > > without a major redesign of the whole large pages mecanism.
    >
    > I don't see much in the COW code which is ppc64-specific. All the hardware
    > needs to do is to provide a way to make the big pages readonly. With a bit
    > of an abstraction for the TLB manipulation in there it should be pretty
    > straightforward.
    >
    > Certainly worth the attempt, no?

    Yes, you have a point. However doing it in a cross-arch way will
    require building more of a shared abstraction about hugepage pte
    entries that exists currently. And that will mean making significant
    changes to all the archs to create that abstraction. I don't know
    enough about the other archs to be confident of debugging such
    changes, but I'll see what I can do.

    That should let the actual handle_mm_fault->hugepage_cow codepath be
    shared. However how the hugepage ptes fit in with the rest of the
    pagetables varies from arch to arch - on ppc64 we're considering
    putting hugepages into their own entirely separate pagetables, even -
    so anything that actually walks the pagetables (like
    copy_hugetlb_page_range()) will still have to be arch-specific.

    --
    David Gibson | For every complex problem there is a
    david AT gibson.dropbear.id.au | solution which is simple, neat and
    | wrong.
    http://www.ozlabs.org/people/dgibson
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