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    SubjectRe: [PATCH 10 of 20] ipath - support for userspace apps using core driver
    On Wed, 15 Mar 2006, Andrew Morton wrote:
    > Roland Dreier <rdreier@cisco.com> wrote:
    >
    > > Or maybe it's just simpler to use vm_insert_page() in the .mmap method
    > > and not try to be fancy with .nopage?
    >
    > One would need to work out what to do with these pages when they're shared,
    > after a fork - a ->nopage() handler would still be needed there, assuming
    > the VMA is marked VM_DONTCOPY. Because we don't copy all the pte's on a
    > VM_DONTCOPY vma at fork()-time. (I think we _could_, but we don't)

    Misapprehension of VM_DONTCOPY addressed in another reply.

    > vm_insert_page() mucks around with rmap-named functions which don't
    > actually do rmap and sports apparently-incorrect comments wrt
    > PageReserved(). I don't know how well-cared-for it is...

    It does seem to have four users intree now, so I hope it works.

    It was a byproduct of when Linus thought he could get away with
    limiting remap_pfn_range, in ways that later proved unsustainable.
    It's a bit surplus to requirements now, but does have those users.

    I'm not keen on it, because I think most drivers actually
    want something slightly different (a remap_vmalloc_pages or a
    remap_highorder_page). But that will emerge later on, in that
    fabled time when I go remove the SetPageReserveds from drivers.

    Yes, there are some out-of-date comments thereabouts: I did correct
    them once, but in one of those patches that Linus rejected.

    insert_page does a page_add_file_rmap to bump the mapcount, yes;
    but whether we ever "reverse map" from page to pte depends on other
    things (page->mapping and prio_tree) certainly not set here, and
    probably not (indeed, hopefully not!) done by any vm_insert_page
    caller. That criticism applies to all the page_add_rmaps and
    page_remove_rmaps: we carried over the name from pte_chain days,
    but the only thing that gets added or removed there now is "1".

    Hugh
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