[PATCH 04/12] powerpc: assert_pte_locked() use pte_offset_map_nolock()

Hugh Dickins
2023-05-29T06:19:25Z

Instead of pte_lockptr(), use the recently added pte_offset_map_nolock()
in assert_pte_locked(). BUG if pte_offset_map_nolock() fails: this is
stricter than the previous implementation, which skipped when pmd_none()
(with a comment on khugepaged collapse transitions): but wouldn't we want
to know, if an assert_pte_locked() caller can be racing such transitions?

This mod might cause new crashes: which either expose my ignorance, or
indicate issues to be fixed, or limit the usage of assert_pte_locked().

Signed-off-by: Hugh Dickins <hughd@google.com>
---
arch/powerpc/mm/pgtable.c | 16 ++++++----------
1 file changed, 6 insertions(+), 10 deletions(-)

diff --git a/arch/powerpc/mm/pgtable.c b/arch/powerpc/mm/pgtable.c
index cb2dcdb18f8e..16b061af86d7 100644
--- a/arch/powerpc/mm/pgtable.c
+++ b/arch/powerpc/mm/pgtable.c
@@ -311,6 +311,8 @@ void assert_pte_locked(struct mm_struct *mm, unsigned long addr)
p4d_t *p4d;
pud_t *pud;
pmd_t *pmd;
+ pte_t *pte;
+ spinlock_t *ptl;

if (mm == &init_mm)
return;
@@ -321,16 +323,10 @@ void assert_pte_locked(struct mm_struct *mm, unsigned long addr)
pud = pud_offset(p4d, addr);
BUG_ON(pud_none(*pud));
pmd = pmd_offset(pud, addr);
- /*
- * khugepaged to collapse normal pages to hugepage, first set
- * pmd to none to force page fault/gup to take mmap_lock. After
- * pmd is set to none, we do a pte_clear which does this assertion
- * so if we find pmd none, return.
- */
- if (pmd_none(*pmd))
- return;
- BUG_ON(!pmd_present(*pmd));
- assert_spin_locked(pte_lockptr(mm, pmd));
+ pte = pte_offset_map_nolock(mm, pmd, addr, &ptl);
+ BUG_ON(!pte);
+ assert_spin_locked(ptl);
+ pte_unmap(pte);
}
#endif /* CONFIG_DEBUG_VM */

--
2.35.3