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    SubjectRe: [PATCH/RFC G-U-P experts] IB/umem: Modernize our get_user_pages() parameters
    > Sigh, what a mess ... it seems what we really want to do is know
    > if userspace might trigger a COW because or not, and only do a
    > preemptive COW in that case.  (We're not really concerned with
    > userspace fork()ing and setting up a COW in the future, since that's
    > what we have MADV_DONTFORK for)
    >
    > The status quo works for userspace anonymous mappings but
    > it doesn't work for my case of mapping a kernel buffer read-only
    > into userspace.  And fixing my case breaks the anonymous case.
    > Do you see a way out of this dilemma?  Do we need to add yet
    > another flag to get_user_pages()?

    So thinking about this a bit more... it seems what we want is at least
    to first order that we do the equivalent of write==1 exactly when the vma
    for a mapping has VM_WRITE set (or is it VMA_MAYWRITE / force==1?
    I don't quite understand the distinction between WRITE and MAYWRITE).

    Right now, one call to get_user_pages() might involve more than one vma,
    but we could simulate the above by doing find_vma() and making sure our
    call to get_user_pages() goes one vma at a time. Of course that would be
    inefficient since get_user_pages() will redo the find_vma() internally, so it
    would I guess make sense to add another FOLL_ flag to tell
    get_user_pages() to do this?

    Am I all wet, or am I becoming an MM hacker?

    Thanks,
    Roland
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