[patch 040/166] userfaultfd: wp: drop _PAGE_UFFD_WP properly when fork

Andrew Morton
2020-04-07T03:05:56Z

From: Peter Xu <peterx@redhat.com>
Subject: userfaultfd: wp: drop _PAGE_UFFD_WP properly when fork

UFFD_EVENT_FORK support for uffd-wp should be already there, except that
we should clean the uffd-wp bit if uffd fork event is not enabled. Detect
that to avoid _PAGE_UFFD_WP being set even if the VMA is not being tracked
by VM_UFFD_WP. Do this for both small PTEs and huge PMDs.

Link: http://lkml.kernel.org/r/20200220163112.11409-9-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Jerome Glisse <jglisse@redhat.com>
Reviewed-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Bobby Powers <bobbypowers@gmail.com>
Cc: Brian Geffon <bgeffon@google.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Denis Plotnikov <dplotnikov@virtuozzo.com>
Cc: "Dr . David Alan Gilbert" <dgilbert@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: "Kirill A . Shutemov" <kirill@shutemov.name>
Cc: Martin Cracauer <cracauer@cons.org>
Cc: Marty McFadden <mcfadden8@llnl.gov>
Cc: Maya Gokhale <gokhale2@llnl.gov>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Pavel Emelyanov <xemul@openvz.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: Shaohua Li <shli@fb.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

mm/huge_memory.c | 8 ++++++++
mm/memory.c | 8 ++++++++
2 files changed, 16 insertions(+)

--- a/mm/huge_memory.c~userfaultfd-wp-drop-_page_uffd_wp-properly-when-fork
+++ a/mm/huge_memory.c
@@ -1044,6 +1044,14 @@ int copy_huge_pmd(struct mm_struct *dst_
ret = -EAGAIN;
pmd = *src_pmd;

+ /*
+ * Make sure the _PAGE_UFFD_WP bit is cleared if the new VMA
+ * does not have the VM_UFFD_WP, which means that the uffd
+ * fork event is not enabled.
+ */
+ if (!(vma->vm_flags & VM_UFFD_WP))
+ pmd = pmd_clear_uffd_wp(pmd);
+
#ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION
if (unlikely(is_swap_pmd(pmd))) {
swp_entry_t entry = pmd_to_swp_entry(pmd);
--- a/mm/memory.c~userfaultfd-wp-drop-_page_uffd_wp-properly-when-fork
+++ a/mm/memory.c
@@ -785,6 +785,14 @@ copy_one_pte(struct mm_struct *dst_mm, s
pte = pte_mkclean(pte);
pte = pte_mkold(pte);

+ /*
+ * Make sure the _PAGE_UFFD_WP bit is cleared if the new VMA
+ * does not have the VM_UFFD_WP, which means that the uffd
+ * fork event is not enabled.
+ */
+ if (!(vm_flags & VM_UFFD_WP))
+ pte = pte_clear_uffd_wp(pte);
+
page = vm_normal_page(vma, addr, pte);
if (page) {
get_page(page);
_