Re: [PATCH next] powerpc/mm: fix _PAGE_SWP_SOFT_DIRTY breaking swapoff

["Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>]

Hugh Dickins <hughd@google.com> writes:

> Swapoff after swapping hangs on the G5, when CONFIG_CHECKPOINT_RESTORE=y
> but CONFIG_MEM_SOFT_DIRTY is not set.  That's because the non-zero
> _PAGE_SWP_SOFT_DIRTY bit, added by CONFIG_HAVE_ARCH_SOFT_DIRTY=y, is not
> discounted when CONFIG_MEM_SOFT_DIRTY is not set: so swap ptes cannot be
> recognized.
>
> (I suspect that the peculiar dependence of HAVE_ARCH_SOFT_DIRTY on
> CHECKPOINT_RESTORE in arch/powerpc/Kconfig comes from an incomplete
> attempt to solve this problem.)
>
> It's true that the relationship between CONFIG_HAVE_ARCH_SOFT_DIRTY and
> and CONFIG_MEM_SOFT_DIRTY is too confusing, and it's true that swapoff
> should be made more robust; but nevertheless, fix up the powerpc ifdefs
> as x86_64 and s390 (which met the same problem) have them, defining the
> bits as 0 if CONFIG_MEM_SOFT_DIRTY is not set.

Do we need this patch, if we make the maybe_same_pte() more robust. The
#ifdef with pte bits is always a confusing one and IMHO, we should avoid
that if we can ?

>
> Signed-off-by: Hugh Dickins <hughd@google.com>
> ---
>
>  arch/powerpc/include/asm/book3s/64/hash.h    |    5 +++++
>  arch/powerpc/include/asm/book3s/64/pgtable.h |    9 ++++++---
>  2 files changed, 11 insertions(+), 3 deletions(-)
>
> --- 4.4-next/arch/powerpc/include/asm/book3s/64/hash.h	2016-01-06 11:54:01.377508976 -0800
> +++ linux/arch/powerpc/include/asm/book3s/64/hash.h	2016-01-09 13:54:24.410893347 -0800
> @@ -33,7 +33,12 @@
>  #define _PAGE_F_GIX_SHIFT	12
>  #define _PAGE_F_SECOND		0x08000 /* Whether to use secondary hash or not */
>  #define _PAGE_SPECIAL		0x10000 /* software: special page */
> +
> +#ifdef CONFIG_MEM_SOFT_DIRTY
>  #define _PAGE_SOFT_DIRTY	0x20000 /* software: software dirty tracking */
> +#else
> +#define _PAGE_SOFT_DIRTY	0x00000
> +#endif
>
>  /*
>   * We need to differentiate between explicit huge page and THP huge
> --- 4.4-next/arch/powerpc/include/asm/book3s/64/pgtable.h	2016-01-06 11:54:01.377508976 -0800
> +++ linux/arch/powerpc/include/asm/book3s/64/pgtable.h	2016-01-09 13:54:24.410893347 -0800
> @@ -162,8 +162,13 @@ static inline void pgd_set(pgd_t *pgdp,
>  #define __pte_to_swp_entry(pte)		((swp_entry_t) { pte_val((pte)) })
>  #define __swp_entry_to_pte(x)		__pte((x).val)
>
> -#ifdef CONFIG_HAVE_ARCH_SOFT_DIRTY
> +#ifdef CONFIG_MEM_SOFT_DIRTY
>  #define _PAGE_SWP_SOFT_DIRTY   (1UL << (SWP_TYPE_BITS + _PAGE_BIT_SWAP_TYPE))
> +#else
> +#define _PAGE_SWP_SOFT_DIRTY	0UL
> +#endif /* CONFIG_MEM_SOFT_DIRTY */
> +
> +#ifdef CONFIG_HAVE_ARCH_SOFT_DIRTY
>  static inline pte_t pte_swp_mksoft_dirty(pte_t pte)
>  {
>  	return __pte(pte_val(pte) | _PAGE_SWP_SOFT_DIRTY);
> @@ -176,8 +181,6 @@ static inline pte_t pte_swp_clear_soft_d
>  {
>  	return __pte(pte_val(pte) & ~_PAGE_SWP_SOFT_DIRTY);
>  }
> -#else
> -#define _PAGE_SWP_SOFT_DIRTY	0
>  #endif /* CONFIG_HAVE_ARCH_SOFT_DIRTY */
>
>  void pgtable_cache_add(unsigned shift, void (*ctor)(void *));