From: Peter Xu <peterx@redhat.com>
To: linux-mm@kvack.org, linux-kernel@vger.kernel.org
Cc: Brian Geffon <bgeffon@google.com>,
Pavel Emelyanov <xemul@virtuozzo.com>,
Mike Kravetz <mike.kravetz@oracle.com>,
David Hildenbrand <david@redhat.com>,
peterx@redhat.com, Martin Cracauer <cracauer@cons.org>,
Andrea Arcangeli <aarcange@redhat.com>,
Mel Gorman <mgorman@suse.de>,
Bobby Powers <bobbypowers@gmail.com>,
Mike Rapoport <rppt@linux.vnet.ibm.com>,
"Kirill A . Shutemov" <kirill@shutemov.name>,
Maya Gokhale <gokhale2@llnl.gov>,
Johannes Weiner <hannes@cmpxchg.org>,
Marty McFadden <mcfadden8@llnl.gov>,
Denis Plotnikov <dplotnikov@virtuozzo.com>,
Hugh Dickins <hughd@google.com>,
"Dr . David Alan Gilbert" <dgilbert@redhat.com>,
Jerome Glisse <jglisse@redhat.com>
Subject: [PATCH v6 16/19] userfaultfd: wp: UFFDIO_REGISTER_MODE_WP documentation update
Date: Thu, 20 Feb 2020 11:31:09 -0500 [thread overview]
Message-ID: <20200220163112.11409-17-peterx@redhat.com> (raw)
In-Reply-To: <20200220163112.11409-1-peterx@redhat.com>
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>
Reviewed-by: Mike Rapoport <rppt@linux.vnet.ibm.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
========
--
2.24.1
next prev parent reply other threads:[~2020-02-20 16:31 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-20 16:30 [PATCH v6 00/19] userfaultfd: write protection support Peter Xu
2020-02-20 16:30 ` [PATCH v6 01/19] userfaultfd: wp: add helper for writeprotect check Peter Xu
2020-02-20 16:30 ` [PATCH v6 02/19] userfaultfd: wp: hook userfault handler to write protection fault Peter Xu
2020-02-20 16:30 ` [PATCH v6 03/19] userfaultfd: wp: add WP pagetable tracking to x86 Peter Xu
2020-02-20 16:30 ` [PATCH v6 04/19] userfaultfd: wp: userfaultfd_pte/huge_pmd_wp() helpers Peter Xu
2020-02-20 16:30 ` [PATCH v6 05/19] userfaultfd: wp: add UFFDIO_COPY_MODE_WP Peter Xu
2020-02-20 16:30 ` [PATCH v6 06/19] mm: merge parameters for change_protection() Peter Xu
2020-02-20 16:31 ` [PATCH v6 07/19] userfaultfd: wp: apply _PAGE_UFFD_WP bit Peter Xu
2020-02-20 16:31 ` [PATCH v6 08/19] userfaultfd: wp: drop _PAGE_UFFD_WP properly when fork Peter Xu
2020-02-20 16:31 ` [PATCH v6 09/19] userfaultfd: wp: add pmd_swp_*uffd_wp() helpers Peter Xu
2020-02-20 16:31 ` [PATCH v6 10/19] userfaultfd: wp: support swap and page migration Peter Xu
2020-02-20 16:31 ` [PATCH v6 11/19] khugepaged: skip collapse if uffd-wp detected Peter Xu
2020-02-20 16:31 ` [PATCH v6 12/19] userfaultfd: wp: support write protection for userfault vma range Peter Xu
2020-02-20 16:31 ` [PATCH v6 13/19] userfaultfd: wp: add the writeprotect API to userfaultfd ioctl Peter Xu
2020-02-20 16:31 ` [PATCH v6 14/19] userfaultfd: wp: enabled write protection in userfaultfd API Peter Xu
2020-02-20 16:31 ` [PATCH v6 15/19] userfaultfd: wp: don't wake up when doing write protect Peter Xu
2020-02-20 16:31 ` Peter Xu [this message]
2020-02-20 16:31 ` [PATCH v6 17/19] userfaultfd: wp: declare _UFFDIO_WRITEPROTECT conditionally Peter Xu
2020-02-20 16:31 ` [PATCH v6 18/19] userfaultfd: selftests: refactor statistics Peter Xu
2020-02-20 16:31 ` [PATCH v6 19/19] userfaultfd: selftests: add write-protect test Peter Xu
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