[PATCH RFC 22/24] userfaultfd: wp: UFFDIO_REGISTER_MODE_WP documentation update

Peter Xu
2019-01-21T08:00:37Z

From: Martin Cracauer <cracauer@cons.org>

Adds documentation about the write protection support.

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
[peterx: rewrite in rst format; fixups here and there]
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