[PATCH 3.16 192/204] mm, thp: Do not make page table dirty unconditionally in touch_p[mu]d()

[Ben Hutchings]

3.16.52-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>

commit a8f97366452ed491d13cf1e44241bc0b5740b1f0 upstream.

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@xxxxxxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
[carnil: backport for 3.16:
 - Adjust context
 - Drop specific part for PUD-sized transparent hugepages. Support
   for PUD-sized transparent hugepages was added in v4.11-rc1
]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 mm/huge_memory.c | 36 +++++++++++++-----------------------
 1 file changed, 13 insertions(+), 23 deletions(-)

--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1230,17 +1230,11 @@ struct page *follow_trans_huge_pmd(struc
 
 	if (flags & FOLL_TOUCH) {
 		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);
 	}
 	if ((flags & FOLL_MLOCK) && (vma->vm_flags & VM_LOCKED)) {