[PATCHv4 10/12] mm: Do not lose dirty and access bits in pmdp_invalidate()

[Kirill A. Shutemov]

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

The patch change pmdp_invalidate() to make the entry non-present atomically and
return previous value of the entry. This value can be used to check if
CPU set dirty/accessed bits under us.

The race window is very small and I haven't seen any reports that can be
attributed to the bug. For this reason, I don't think backporting to
stable trees needed.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Reported-by: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
---
 include/asm-generic/pgtable.h | 2 +-
 mm/pgtable-generic.c          | 6 +++---
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/include/asm-generic/pgtable.h b/include/asm-generic/pgtable.h
index ae83b14200b8..f449c71cbdc0 100644
--- a/include/asm-generic/pgtable.h
+++ b/include/asm-generic/pgtable.h
@@ -325,7 +325,7 @@ static inline pmd_t generic_pmdp_establish(struct vm_area_struct *vma,
 #endif
 
 #ifndef __HAVE_ARCH_PMDP_INVALIDATE
-extern void pmdp_invalidate(struct vm_area_struct *vma, unsigned long address,
+extern pmd_t pmdp_invalidate(struct vm_area_struct *vma, unsigned long address,
 			    pmd_t *pmdp);
 #endif
 
diff --git a/mm/pgtable-generic.c b/mm/pgtable-generic.c
index 1e4ee763c190..cf2af04b34b9 100644
--- a/mm/pgtable-generic.c
+++ b/mm/pgtable-generic.c
@@ -181,12 +181,12 @@ pgtable_t pgtable_trans_huge_withdraw(struct mm_struct *mm, pmd_t *pmdp)
 #endif
 
 #ifndef __HAVE_ARCH_PMDP_INVALIDATE
-void pmdp_invalidate(struct vm_area_struct *vma, unsigned long address,
+pmd_t 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));
+	pmd_t old = pmdp_establish(vma, address, pmdp, pmd_mknotpresent(*pmdp));
 	flush_pmd_tlb_range(vma, address, address + HPAGE_PMD_SIZE);
+	return old;
 }
 #endif
 
-- 
2.15.0