[PATCH next] mm: make swapoff more robust against soft dirty

[Aneesh Kumar K.V]

Hugh Dickins <hughd at google.com> writes:

> Both s390 and powerpc have hit the issue of swapoff hanging, when
> CONFIG_HAVE_ARCH_SOFT_DIRTY and CONFIG_MEM_SOFT_DIRTY ifdefs were
> not quite as x86_64 had them.  I think it would be much clearer if
> HAVE_ARCH_SOFT_DIRTY was just a Kconfig option set by architectures
> to determine whether the MEM_SOFT_DIRTY option should be offered,
> and the actual code depend upon CONFIG_MEM_SOFT_DIRTY alone.
>
> But won't embark on that change myself: instead make swapoff more
> robust, by using pte_swp_clear_soft_dirty() on each pte it encounters,
> without an explicit #ifdef CONFIG_MEM_SOFT_DIRTY.  That being a no-op,
> whether the bit in question is defined as 0 or the asm-generic fallback
> is used, unless soft dirty is fully turned on.
>
> Why "maybe" in maybe_same_pte()?  Rename it pte_same_as_swp().
>

Ok this also explains, the _PAGE_PTE issue on powerpc you mentioned in the other
email.

Reviewed-by: Aneesh Kumar K.V <aneesh.kumar at linux.vnet.ibm.com>

> Signed-off-by: Hugh Dickins <hughd at google.com>
> ---
>
>  mm/swapfile.c |   18 ++++--------------
>  1 file changed, 4 insertions(+), 14 deletions(-)
>
> --- 4.4-next/mm/swapfile.c	2016-01-06 11:54:46.327006983 -0800
> +++ linux/mm/swapfile.c	2016-01-09 13:39:19.632872694 -0800
> @@ -1109,19 +1109,9 @@ unsigned int count_swap_pages(int type,
>  }
>  #endif /* CONFIG_HIBERNATION */
>
> -static inline int maybe_same_pte(pte_t pte, pte_t swp_pte)
> +static inline int pte_same_as_swp(pte_t pte, pte_t swp_pte)
>  {
> -#ifdef CONFIG_MEM_SOFT_DIRTY
> -	/*
> -	 * When pte keeps soft dirty bit the pte generated
> -	 * from swap entry does not has it, still it's same
> -	 * pte from logical point of view.
> -	 */
> -	pte_t swp_pte_dirty = pte_swp_mksoft_dirty(swp_pte);
> -	return pte_same(pte, swp_pte) || pte_same(pte, swp_pte_dirty);
> -#else
> -	return pte_same(pte, swp_pte);
> -#endif
> +	return pte_same(pte_swp_clear_soft_dirty(pte), swp_pte);
>  }
>
>  /*
> @@ -1150,7 +1140,7 @@ static int unuse_pte(struct vm_area_stru
>  	}
>
>  	pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);
> -	if (unlikely(!maybe_same_pte(*pte, swp_entry_to_pte(entry)))) {
> +	if (unlikely(!pte_same_as_swp(*pte, swp_entry_to_pte(entry)))) {
>  		mem_cgroup_cancel_charge(page, memcg, false);
>  		ret = 0;
>  		goto out;
> @@ -1208,7 +1198,7 @@ static int unuse_pte_range(struct vm_are
>  		 * swapoff spends a _lot_ of time in this loop!
>  		 * Test inline before going to call unuse_pte.
>  		 */
> -		if (unlikely(maybe_same_pte(*pte, swp_pte))) {
> +		if (unlikely(pte_same_as_swp(*pte, swp_pte))) {
>  			pte_unmap(pte);
>  			ret = unuse_pte(vma, pmd, addr, entry, page);
>  			if (ret)