s390/mm: fix set_huge_pte_at() for empty ptes

[ Upstream commit ac8372f3b4e41015549b331a4f350224661e7fc6 ]

On s390, the layout of normal and large ptes (i.e. pmds/puds) differs.
Therefore, set_huge_pte_at() does a conversion from a normal pte to
the corresponding large pmd/pud. So, when converting an empty pte, this
should result in an empty pmd/pud, which would return true for

However, after conversion we also mark the pmd/pud as large, and
therefore present. For empty ptes, this will result in an empty pmd/pud
that is also marked as large, and pmd/pud_none() would not return true.

There is currently no issue with this behaviour, as set_huge_pte_at()
does not seem to be called for empty ptes. It would be valid though, so
let's fix this by not marking empty ptes as large in set_huge_pte_at().

This was found by testing a patch from from Anshuman Khandual, which is
currently discussed on LKML ("mm/debug: Add more arch page table helper

Signed-off-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
diff --git a/arch/s390/mm/hugetlbpage.c b/arch/s390/mm/hugetlbpage.c
index 5674710..7dfae86 100644
--- a/arch/s390/mm/hugetlbpage.c
+++ b/arch/s390/mm/hugetlbpage.c
@@ -159,10 +159,13 @@
 	/* Set correct table type for 2G hugepages */
-	if ((pte_val(*ptep) & _REGION_ENTRY_TYPE_MASK) == _REGION_ENTRY_TYPE_R3)
-	else
+	if ((pte_val(*ptep) & _REGION_ENTRY_TYPE_MASK) == _REGION_ENTRY_TYPE_R3) {
+		if (likely(pte_present(pte)))
+			rste |= _REGION3_ENTRY_LARGE;
+		rste |= _REGION_ENTRY_TYPE_R3;
+	} else if (likely(pte_present(pte)))
 	clear_huge_pte_skeys(mm, rste);
 	pte_val(*ptep) = rste;