mm, thp: Do not make page table dirty unconditionally in touch_p[mu]d()

Currently, we unconditionally make page table dirty in touch_pmd().
It may result in false-positive can_follow_write_pmd().

We may avoid the situation, if we would only make the page table entry
dirty if caller asks for write access -- FOLL_WRITE.

The patch also changes touch_pud() in the same way.

Signed-off-by: Kirill A. Shutemov <[e-mail]>
Cc: Michal Hocko <[e-mail]>
Cc: Hugh Dickins <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/huge_memory.c

···
 #endif /* CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD */
 
 static void touch_pmd(struct vm_area_struct *vma, unsigned long addr,
-		pmd_t *pmd)
+		pmd_t *pmd, int flags)
 {
 	pmd_t _pmd;
 
-	/*
-	 * We should set the dirty bit only for FOLL_WRITE but for now
-	 * the dirty bit in the pmd is meaningless.  And if the dirty
-	 * bit will become meaningful and we'll only set it with
-	 * FOLL_WRITE, an atomic set_bit will be required on the pmd to
-	 * set the young bit, instead of the current set_pmd_at.
-	 */
-	_pmd = pmd_mkyoung(pmd_mkdirty(*pmd));
+	_pmd = pmd_mkyoung(*pmd);
+	if (flags & FOLL_WRITE)
+		_pmd = pmd_mkdirty(_pmd);
 	if (pmdp_set_access_flags(vma, addr & HPAGE_PMD_MASK,
-				pmd, _pmd,  1))
+				pmd, _pmd, flags & FOLL_WRITE))
 		update_mmu_cache_pmd(vma, addr, pmd);
 }
 
···
 		return NULL;
 
 	if (flags & FOLL_TOUCH)
-		touch_pmd(vma, addr, pmd);
+		touch_pmd(vma, addr, pmd, flags);
 
 	/*
 	 * device mapped pages can only be returned if the
···
 
 #ifdef CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD
 static void touch_pud(struct vm_area_struct *vma, unsigned long addr,
-		pud_t *pud)
+		pud_t *pud, int flags)
 {
 	pud_t _pud;
 
-	/*
-	 * We should set the dirty bit only for FOLL_WRITE but for now
-	 * the dirty bit in the pud is meaningless.  And if the dirty
-	 * bit will become meaningful and we'll only set it with
-	 * FOLL_WRITE, an atomic set_bit will be required on the pud to
-	 * set the young bit, instead of the current set_pud_at.
-	 */
-	_pud = pud_mkyoung(pud_mkdirty(*pud));
+	_pud = pud_mkyoung(*pud);
+	if (flags & FOLL_WRITE)
+		_pud = pud_mkdirty(_pud);
 	if (pudp_set_access_flags(vma, addr & HPAGE_PUD_MASK,
-				pud, _pud,  1))
+				pud, _pud, flags & FOLL_WRITE))
 		update_mmu_cache_pud(vma, addr, pud);
 }
 
···
 		return NULL;
 
 	if (flags & FOLL_TOUCH)
-		touch_pud(vma, addr, pud);
+		touch_pud(vma, addr, pud, flags);
 
 	/*
 	 * device mapped pages can only be returned if the
···
 	page = pmd_page(*pmd);
 	VM_BUG_ON_PAGE(!PageHead(page) && !is_zone_device_page(page), page);
 	if (flags & FOLL_TOUCH)
-		touch_pmd(vma, addr, pmd);
+		touch_pmd(vma, addr, pmd, flags);
 	if ((flags & FOLL_MLOCK) && (vma->vm_flags & VM_LOCKED)) {
 		/*
 		 * We don't mlock() pte-mapped THPs. This way we can avoid