[PATCH 1/2] mm: vm_page_prot: update with WRITE_ONCE/READ_ONCE

[Andrea Arcangeli <aarcange@redhat.com>]

vma->vm_page_prot is read lockless from the rmap_walk, it may be
updated concurrently and this prevents the risk of reading
intermediate values.

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
---
 mm/huge_memory.c | 2 +-
 mm/migrate.c     | 2 +-
 mm/mmap.c        | 9 ++++++---
 3 files changed, 8 insertions(+), 5 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index cb95a83..995f8a1 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1566,7 +1566,7 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
 			if (soft_dirty)
 				entry = pte_swp_mksoft_dirty(entry);
 		} else {
-			entry = mk_pte(page + i, vma->vm_page_prot);
+			entry = mk_pte(page + i, READ_ONCE(vma->vm_page_prot));
 			entry = maybe_mkwrite(entry, vma);
 			if (!write)
 				entry = pte_wrprotect(entry);
diff --git a/mm/migrate.c b/mm/migrate.c
index 00167df..e49ccce 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -234,7 +234,7 @@ static int remove_migration_pte(struct page *new, struct vm_area_struct *vma,
 		goto unlock;
 
 	get_page(new);
-	pte = pte_mkold(mk_pte(new, vma->vm_page_prot));
+	pte = pte_mkold(mk_pte(new, READ_ONCE(vma->vm_page_prot)));
 	if (pte_swp_soft_dirty(*ptep))
 		pte = pte_mksoft_dirty(pte);
 
diff --git a/mm/mmap.c b/mm/mmap.c
index c34f643..1abf106 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -112,13 +112,16 @@ static pgprot_t vm_pgprot_modify(pgprot_t oldprot, unsigned long vm_flags)
 void vma_set_page_prot(struct vm_area_struct *vma)
 {
 	unsigned long vm_flags = vma->vm_flags;
+	pgprot_t vm_page_prot;
 
-	vma->vm_page_prot = vm_pgprot_modify(vma->vm_page_prot, vm_flags);
+	vm_page_prot = vm_pgprot_modify(vma->vm_page_prot, vm_flags);
 	if (vma_wants_writenotify(vma)) {
 		vm_flags &= ~VM_SHARED;
-		vma->vm_page_prot = vm_pgprot_modify(vma->vm_page_prot,
-						     vm_flags);
+		vm_page_prot = vm_pgprot_modify(vma->vm_page_prot,
+						vm_flags);
 	}
+	/* remove_protection_ptes reads vma->vm_page_prot without mmap_sem */
+	WRITE_ONCE(vma->vm_page_prot, vm_page_prot);
 }
 
 static unsigned long vm_mergeable __read_mostly;

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