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    Subject[PATCH v3 24/28] userfaultfd: wp: UFFDIO_REGISTER_MODE_WP documentation update
    Date
    From: Martin Cracauer <cracauer@cons.org>

    Adds documentation about the write protection support.

    Signed-off-by: Martin Cracauer <cracauer@cons.org>
    Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
    [peterx: rewrite in rst format; fixups here and there]
    Reviewed-by: Jerome Glisse <jglisse@redhat.com>
    Signed-off-by: Peter Xu <peterx@redhat.com>
    ---
    Documentation/admin-guide/mm/userfaultfd.rst | 51 ++++++++++++++++++++
    1 file changed, 51 insertions(+)

    diff --git a/Documentation/admin-guide/mm/userfaultfd.rst b/Documentation/admin-guide/mm/userfaultfd.rst
    index 5048cf661a8a..c30176e67900 100644
    --- a/Documentation/admin-guide/mm/userfaultfd.rst
    +++ b/Documentation/admin-guide/mm/userfaultfd.rst
    @@ -108,6 +108,57 @@ UFFDIO_COPY. They're atomic as in guaranteeing that nothing can see an
    half copied page since it'll keep userfaulting until the copy has
    finished.

    +Notes:
    +
    +- If you requested UFFDIO_REGISTER_MODE_MISSING when registering then
    + you must provide some kind of page in your thread after reading from
    + the uffd. You must provide either UFFDIO_COPY or UFFDIO_ZEROPAGE.
    + The normal behavior of the OS automatically providing a zero page on
    + an annonymous mmaping is not in place.
    +
    +- None of the page-delivering ioctls default to the range that you
    + registered with. You must fill in all fields for the appropriate
    + ioctl struct including the range.
    +
    +- You get the address of the access that triggered the missing page
    + event out of a struct uffd_msg that you read in the thread from the
    + uffd. You can supply as many pages as you want with UFFDIO_COPY or
    + UFFDIO_ZEROPAGE. Keep in mind that unless you used DONTWAKE then
    + the first of any of those IOCTLs wakes up the faulting thread.
    +
    +- Be sure to test for all errors including (pollfd[0].revents &
    + POLLERR). This can happen, e.g. when ranges supplied were
    + incorrect.
    +
    +Write Protect Notifications
    +---------------------------
    +
    +This is equivalent to (but faster than) using mprotect and a SIGSEGV
    +signal handler.
    +
    +Firstly you need to register a range with UFFDIO_REGISTER_MODE_WP.
    +Instead of using mprotect(2) you use ioctl(uffd, UFFDIO_WRITEPROTECT,
    +struct *uffdio_writeprotect) while mode = UFFDIO_WRITEPROTECT_MODE_WP
    +in the struct passed in. The range does not default to and does not
    +have to be identical to the range you registered with. You can write
    +protect as many ranges as you like (inside the registered range).
    +Then, in the thread reading from uffd the struct will have
    +msg.arg.pagefault.flags & UFFD_PAGEFAULT_FLAG_WP set. Now you send
    +ioctl(uffd, UFFDIO_WRITEPROTECT, struct *uffdio_writeprotect) again
    +while pagefault.mode does not have UFFDIO_WRITEPROTECT_MODE_WP set.
    +This wakes up the thread which will continue to run with writes. This
    +allows you to do the bookkeeping about the write in the uffd reading
    +thread before the ioctl.
    +
    +If you registered with both UFFDIO_REGISTER_MODE_MISSING and
    +UFFDIO_REGISTER_MODE_WP then you need to think about the sequence in
    +which you supply a page and undo write protect. Note that there is a
    +difference between writes into a WP area and into a !WP area. The
    +former will have UFFD_PAGEFAULT_FLAG_WP set, the latter
    +UFFD_PAGEFAULT_FLAG_WRITE. The latter did not fail on protection but
    +you still need to supply a page when UFFDIO_REGISTER_MODE_MISSING was
    +used.
    +
    QEMU/KVM
    ========

    --
    2.17.1
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