[PATCH 2/3] mm: Do not loose dirty and access bits in pmdp_invalidate()

[Kirill A. Shutemov]

Vlastimil noted that pmdp_invalidate() is not atomic and we can loose
dirty and access bits if CPU sets them after pmdp dereference, but
before set_pmd_at().

The bug doesn't lead to user-visible misbehaviour in current kernel.

Loosing access bit can lead to sub-optimal reclaim behaviour for THP,
but nothing destructive.

Loosing dirty bit is not a big deal too: we would make page dirty
unconditionally on splitting huge page.

The fix is critical for future work on THP: both huge-ext4 and THP swap
out rely on proper dirty tracking.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Reported-by: Vlastimil Babka <vbabka@xxxxxxx>
---
 mm/pgtable-generic.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/mm/pgtable-generic.c b/mm/pgtable-generic.c
index c99d9512a45b..68094fa190d1 100644
--- a/mm/pgtable-generic.c
+++ b/mm/pgtable-generic.c
@@ -182,8 +182,7 @@ pgtable_t pgtable_trans_huge_withdraw(struct mm_struct *mm, pmd_t *pmdp)
 void pmdp_invalidate(struct vm_area_struct *vma, unsigned long address,
 		     pmd_t *pmdp)
 {
-	pmd_t entry = *pmdp;
-	set_pmd_at(vma->vm_mm, address, pmdp, pmd_mknotpresent(entry));
+	pmdp_mknotpresent(pmdp);
 	flush_pmd_tlb_range(vma, address, address + HPAGE_PMD_SIZE);
 }
 #endif
-- 
2.11.0