Re: [PATCH v2 24/26] userfaultfd: wp: UFFDIO_REGISTER_MODE_WP documentation update

[Jerome Glisse]

On Tue, Feb 12, 2019 at 10:56:30AM +0800, Peter Xu wrote:
> From: Martin Cracauer <cracauer@xxxxxxxx>
> 
> Adds documentation about the write protection support.
> 
> Signed-off-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> [peterx: rewrite in rst format; fixups here and there]
> Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>

Reviewed-by: Jérôme Glisse <jglisse@xxxxxxxxxx>

> ---
>  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
>