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

[ Upstream 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: Sasha Levin <alexander.levin@verizon.com>

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index bdd6a8dd57973037304a603d0bacb6c3fbe5bc27..4aa31c77cbab424350221ccd360a8ef3f3b86000 100644 (file)
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1235,17 +1235,11 @@ struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
        VM_BUG_ON_PAGE(!PageHead(page), page);
        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_POPULATE) && (vma->vm_flags & VM_LOCKED)) {