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
