[RFC PATCH 02/14] userfaultfd: try to map write-unprotected pages

[Nadav Amit]

From: Nadav Amit <namit@xxxxxxxxxx>

When using userfaultfd write-(un)protect ioctl, try to change the PTE to
be writable. This would save a page-fault afterwards.

Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: David Hildenbrand <david@xxxxxxxxxx>
Cc: Peter Xu <peterx@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Will Deacon <will@xxxxxxxxxx>
Cc: Yu Zhao <yuzhao@xxxxxxxxxx>
Cc: Nick Piggin <npiggin@xxxxxxxxx>
Signed-off-by: Nadav Amit <namit@xxxxxxxxxx>
---
 mm/userfaultfd.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index e0492f5f06a0..6013b217e9f3 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -799,6 +799,8 @@ int mwriteprotect_range(struct mm_struct *dst_mm, unsigned long start,
 	cp_flags = enable_wp ? MM_CP_UFFD_WP : MM_CP_UFFD_WP_RESOLVE;
 	if (uffd_flags & (UFFD_FLAGS_ACCESS_LIKELY|UFFD_FLAGS_WRITE_LIKELY))
 		cp_flags |= MM_CP_WILL_NEED;
+	if (!enable_wp && (uffd_flags & UFFD_FLAGS_WRITE_LIKELY))
+		cp_flags |= MM_CP_TRY_CHANGE_WRITABLE;
 
 	tlb_gather_mmu(&tlb, dst_mm);
 	change_protection(&tlb, dst_vma, start, start + len, newprot, cp_flags);
-- 
2.25.1