[PATCH v2 15/26] userfaultfd: wp: drop _PAGE_UFFD_WP properly when fork

[Peter Xu]

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@xxxxxxxxxx>
---
 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 817335b443c2..fb2234cb595a 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -938,6 +938,14 @@ 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 b5d67bafae35..c2035539e9fd 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