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    SubjectRe: [PATCH/RFC G-U-P experts] IB/umem: Modernize our get_user_pages() parameters
    Hi Andrea, sorry for the slow reply, had to work on other stuff for a bit.

    On Mon, Jan 30, 2012 at 12:20 PM, Andrea Arcangeli <> wrote:
    > If you map it with an mmap(PROT_READ|PROT_WRITE), force or not force
    > won't change a thing in terms of cows. Just make sure you map your
    > control memory right, then safely remove force=1 and you won't get the
    > control page cowed by mistake. Then if you map it with MAP_SHARED it
    > won't be mapped read-only by fork() (leading to either parent or child
    > losing the control on the device), Hugh already suggested you to use
    > MAP_SHARED instead of MAP_PRIVATE.

    Actually this isn't about control memory for the RDMA adapter...
    as you mentioned that typically is MMIO and mapped with remap_pfn
    stuff, without using any GUP stuff.

    I'm talking about the registration of other memory for reading/writing
    by a remote system via RDMA.

    The reason I'm talking about exporting kernel memory is that I wanted
    to do a debugging trick where a kernel module exposed some state
    into an mmap'able buffer. And I wanted to be able to read that state
    even if my broken module killed the whole system (in fact exactly
    when things crash I want to be able to read the state to figure out
    why I crashed!).

    So I wrote a trivial userspace program that does nothing but mmap
    the buffer, accept RDMA connections from remote systems, and
    map the buffer for reading over those connections. Then I can have
    a second system that connects to that process and polls the buffer.

    Because all the RDMA state is setup in advance, I can keep polling
    even after the first system panics. It's sort of like that firewire remote
    debugging, except I only get access to a limited memory buffer.

    The only difficulty is the problem that started this thread, ie a bogus
    COW so the remote system ends up polling the wrong pages. So with
    my original patch, I'm able to debug but I guess we agree it's the
    wrong fix for the general problem, and I'll write up a patch that adds
    what I think is the correct fix (the new FOLL flag) soon.

    - R.

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      Last update: 2012-02-06 18:49    [W:0.020 / U:5.400 seconds]
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