mm/uffd: always wr-protect pte in pte|pmd_mkuffd_wp()

This patch is a cleanup to always wr-protect pte/pmd in mkuffd_wp paths.

The reasons I still think this patch is worthwhile, are:

  (1) It is a cleanup already; diffstat tells.

  (2) It just feels natural after I thought about this, if the pte is uffd
      protected, let's remove the write bit no matter what it was.

  (2) Since x86 is the only arch that supports uffd-wp, it also redefines
      pte|pmd_mkuffd_wp() in that it should always contain removals of
      write bits.  It means any future arch that want to implement uffd-wp
      should naturally follow this rule too.  It's good to make it a
      default, even if with vm_page_prot changes on VM_UFFD_WP.

  (3) It covers more than vm_page_prot.  So no chance of any potential
      future "accident" (like pte_mkdirty() sparc64 or loongarch, even
      though it just got its pte_mkdirty fixed <1 month ago).  It'll be
      fairly clear when reading the code too that we don't worry anything
      before a pte_mkuffd_wp() on uncertainty of the write bit.

We may call pte_wrprotect() one more time in some paths (e.g.  thp split),
but that should be fully local bitop instruction so the overhead should be
negligible.

Although this patch should logically also fix all the known issues on
uffd-wp too recently on page migration (not for numa hint recovery - that
may need another explcit pte_wrprotect), but this is not the plan for that
fix.  So no fixes, and stable doesn't need this.

Link: https://lkml.kernel.org/r/20221214201533.1774616-1-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Ives van Hoorne <ives@codesandbox.io>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Nadav Amit <nadav.amit@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 8 insertions, 8 deletions

diff --git a/include/asm-generic/hugetlb.h b/include/asm-generic/hugetlb.h
index a57d667addd2..d7f6335d3999 100644
--- a/include/asm-generic/hugetlb.h
+++ b/include/asm-generic/hugetlb.h
@@ -25,6 +25,13 @@ static inline pte_t huge_pte_mkwrite(pte_t pte)
 	return pte_mkwrite(pte);
 }
 
+#ifndef __HAVE_ARCH_HUGE_PTE_WRPROTECT
+static inline pte_t huge_pte_wrprotect(pte_t pte)
+{
+	return pte_wrprotect(pte);
+}
+#endif
+
 static inline pte_t huge_pte_mkdirty(pte_t pte)
 {
 	return pte_mkdirty(pte);
@@ -37,7 +44,7 @@ static inline pte_t huge_pte_modify(pte_t pte, pgprot_t newprot)
 
 static inline pte_t huge_pte_mkuffd_wp(pte_t pte)
 {
-	return pte_mkuffd_wp(pte);
+	return huge_pte_wrprotect(pte_mkuffd_wp(pte));
 }
 
 static inline pte_t huge_pte_clear_uffd_wp(pte_t pte)
@@ -104,13 +111,6 @@ static inline int huge_pte_none_mostly(pte_t pte)
 	return huge_pte_none(pte) || is_pte_marker(pte);
 }
 
-#ifndef __HAVE_ARCH_HUGE_PTE_WRPROTECT
-static inline pte_t huge_pte_wrprotect(pte_t pte)
-{
-	return pte_wrprotect(pte);
-}
-#endif
-
 #ifndef __HAVE_ARCH_PREPARE_HUGEPAGE_RANGE
 static inline int prepare_hugepage_range(struct file *file,
 		unsigned long addr, unsigned long len)