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    SubjectRe: [kvm-devel] [PATCH] export notifier #1
    Christoph Lameter wrote:
    > Ahhh. Good to hear. But we will still end in a situation where only
    > the remote ptes point to the page. Maybe the remote instance will dirty
    > the page at that point?
    >
    >

    When the spte is dropped, its dirty bit is transferred to the page.
    >
    >> sharing code, and for you missing a single notifier means memory
    >> corruption because you don't bump the page count to represent the
    >> external reference).
    >>
    >
    > The approach with the export notifier is page based not based on the
    > mm_struct. We only need a single page count for a page that is exported to
    > a number of remote instances of linux. The page count is dropped when all
    > the remote instances have unmapped the page.
    >

    That won't work for kvm. If we have a hundred virtual machines, that
    means 99 no-op notifications.

    Also, our rmap key for finding the spte is keyed on (mm, va). I imagine
    most RDMA cards are similar.
    >
    >
    >>> @@ -966,6 +973,9 @@ int try_to_unmap(struct page *page, int
    >>>
    >>> BUG_ON(!PageLocked(page));
    >>>
    >>> + if (unlikely(PageExported(page)))
    >>> + export_notifier(invalidate_page, page);
    >>> +
    >>>
    >> Passing the page here will complicate things especially for shared
    >> pages across different VM that are already working in KVM. For non
    >>
    >
    > How?
    >
    >
    >> shared pages we could cache the userland mapping address in
    >> page->private but it's a kludge only working for non-shared
    >> pages. Walking twice the anon_vma lists when only a single walk is
    >>
    >
    > There is only the need to walk twice for pages that are marked Exported.
    > And the double walk is only necessary if the exporter does not have its
    > own rmap. The cross partition thing that we are doing has such an rmap and
    > its a matter of walking the exporters rmap to clear out the external
    > references and then we walk the local rmaps. All once.
    >

    The problem is that external mmus need a reverse mapping structure to
    locate their ptes. We can't expand struct page so we need to base it on
    mm + va.

    >
    >> Besides the pinned pages ram leak by having the zero locking window
    >> above I'm curious how you are going to take care of the finegrined
    >> aging that I'm doing with the accessed bit set by hardware in the spte
    >>
    >
    > I think I explained that above. Remote users effectively are forbidden to
    > establish new references to the page by the clearing of the exported bit.
    >
    >

    Can they wait on that bit?


    --
    error compiling committee.c: too many arguments to function



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