[PATCH RFC 17/24] userfaultfd: wp: drop _PAGE_UFFD_WP properly when fork

Peter Xu
2019-01-21T07:59:50Z

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.

Signed-off-by: Peter Xu <peterx@redhat.com>
---
mm/huge_memory.c | 8 ++++++++
mm/memory.c | 8 ++++++++
2 files changed, 16 insertions(+)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 169795c8e56c..2a3ec62e83b6 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -928,6 +928,14 @@ int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm,
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);
diff --git a/mm/memory.c b/mm/memory.c
index a3de13b728f4..f5497752d2a3 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -788,6 +788,14 @@ copy_one_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm,
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);
--
2.17.1