Subject: [RFC PATCH] powerpc/64s: __find_linux_pte synchronization vs pmdp_invalidate

From: Nicholas Piggin <[email-masked]>
Date: Mon, 3 Jun 2019 16:44:08 +1000

The pmd_none check does not catch hugepage collapse, nor does the
pmd_present check in pmd_trans_huge, because hugepage collapse sets
!_PAGE_PRESENT && _PAGE_INVALID (which results in !pmd_none and
pmd_present).

Aneesh noticed we might need this check as well.

---
 arch/powerpc/mm/pgtable.c | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/arch/powerpc/mm/pgtable.c b/arch/powerpc/mm/pgtable.c
index db4a6253df92..7a702d21400a 100644
--- a/arch/powerpc/mm/pgtable.c
+++ b/arch/powerpc/mm/pgtable.c
@@ -372,13 +372,20 @@ pte_t *__find_linux_pte(pgd_t *pgdir, unsigned long ea,
 	pdshift = PMD_SHIFT;
 	pmdp = pmd_offset(&pud, ea);
 	pmd  = READ_ONCE(*pmdp);
-	/*
-	 * A hugepage collapse is captured by pmd_none, because
-	 * it mark the pmd none and do a hpte invalidate.
-	 */
+
 	if (pmd_none(pmd))
 		return NULL;
 
+#ifdef CONFIG_PPC_BOOK3S_64
+	if (pmd_val(pmd) & (_PAGE_PRESENT|_PAGE_INVALID) == _PAGE_INVALID) {
+		/*
+		 * A hugepage collapse is captured by this condition, see
+		 * pmdp_invalidate.
+		 */
+		return NULL;
+	}
+#endif
+
 	if (pmd_trans_huge(pmd) || pmd_devmap(pmd)) {
 		if (is_thp)
 			*is_thp = true;
-- 
2.20.1