[PATCH next] powerpc/mm: fix _PAGE_PTE breaking swapoff

[Hugh Dickins]

Swapoff after swapping hangs on the G5.  That's because the _PAGE_PTE
bit, added by set_pte_at(), is not expected by swapoff: so swap ptes
cannot be recognized.

I'm not sure whether a swap pte should or should not have _PAGE_PTE set:
this patch assumes not, and fixes set_pte_at() to set _PAGE_PTE only on
present entries.

But if that's wrong, a reasonable alternative would be to
#define __pte_to_swp_entry(pte)	((swp_entry_t) { pte_val(pte) & ~_PAGE_PTE })
#define __swp_entry_to_pte(x)	__pte((x).val | _PAGE_PTE)

Signed-off-by: Hugh Dickins <hughd at google.com>
---

 arch/powerpc/mm/pgtable.c |    5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

--- 4.4-next/arch/powerpc/mm/pgtable.c	2016-01-06 11:54:01.477512251 -0800
+++ linux/arch/powerpc/mm/pgtable.c	2016-01-09 13:51:15.793485717 -0800
@@ -180,9 +180,10 @@ void set_pte_at(struct mm_struct *mm, un
 	VM_WARN_ON((pte_val(*ptep) & (_PAGE_PRESENT | _PAGE_USER)) ==
 		(_PAGE_PRESENT | _PAGE_USER));
 	/*
-	 * Add the pte bit when tryint set a pte
+	 * Add the pte bit when setting a pte (not a swap entry)
 	 */
-	pte = __pte(pte_val(pte) | _PAGE_PTE);
+	if (pte_val(pte) & _PAGE_PRESENT)
+		pte = __pte(pte_val(pte) | _PAGE_PTE);
 
 	/* Note: mm->context.id might not yet have been assigned as
 	 * this context might not have been activated yet when this