From c1a9d624242efaea666866abca8146d16ea2a436 Mon Sep 17 00:00:00 2001
From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Date: Mon, 27 Nov 2017 06:21:25 +0300
Subject: [PATCH] mm, thp: Do not make page table dirty unconditionally in
 touch_p[mu]d()

References: git fixes (mm/gup)
Patch-mainline: v4.15
Git-commit: a8f97366452ed491d13cf1e44241bc0b5740b1f0

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 <kirill.shutemov@linux.intel.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Mel Gorman <mgorman@suse.de>
---
 mm/huge_memory.c | 36 +++++++++++++-----------------------
 1 file changed, 13 insertions(+), 23 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index ad2bd7045f8d..04bc9fe1eb58 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -884,20 +884,15 @@ EXPORT_SYMBOL_GPL(vmf_insert_pfn_pud);
 #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);
 }
 
@@ -926,7 +921,7 @@ struct page *follow_devmap_pmd(struct vm_area_struct *vma, unsigned long addr,
 		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
@@ -1015,20 +1010,15 @@ int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm,
 
 #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);
 }
 
@@ -1051,7 +1041,7 @@ struct page *follow_devmap_pud(struct vm_area_struct *vma, unsigned long addr,
 		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
@@ -1413,7 +1403,7 @@ struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
 	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