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    SubjectRe: [PATCH/RFC G-U-P experts] IB/umem: Modernize our get_user_pages() parameters
    On Fri, 27 Jan 2012, Roland Dreier wrote:

    > > 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

    My first impression is that that's not what you want at all: that will
    not do a preliminary COW of anonymous page to be written into by the
    driver when the user only wants VM_READ access. But perhaps I'm
    worrying about the second order while you're sticking to first order.

    Or perhaps I'm misunderstanding the context in which you want to do
    this: are you now accepting to do a different get_user_pages in the
    anonymous and driver-memory cases, and this suggestion was for the
    driver-memory case only?

    > (or is it VMA_MAYWRITE / force==1?
    > I don't quite understand the distinction between WRITE and MAYWRITE).

    I may have told you more than you wanted to know in the other mail.

    > 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?

    I cannot go further, without explanation for why you need get_user_pages
    in the driver-memory case at all.

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

    Certainly more than I'll ever be an RDMA hacker,

     \ /
      Last update: 2012-01-28 03:35    [W:0.022 / U:10.992 seconds]
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