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

[Aneesh Kumar K.V]

Hugh Dickins <hughd at google.com> writes:

> 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.

One of the reason we added _PAGE_PTE is to enable HUGETLB migration. So
we want migratio ptes to have _PAGE_PTE set.

>
> 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)
>

We do clear _PAGE_PTE bits, when converting swp_entry_t to type and
offset. Can you share the stack trace for the hang, which will help me
understand this more ? . 

> 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

-aneesh