linux-kernel - Re: [PATCH] mm/swap: Fix pte_same_as_swp() not removing uffd-wp bit when compare

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Date:   Thu, 3 Jun 2021 20:26:02 -0700 (PDT)
From:   Hugh Dickins <hughd@...gle.com>
To:     Peter Xu <peterx@...hat.com>
cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: [PATCH] mm/swap: Fix pte_same_as_swp() not removing uffd-wp bit
 when compare

On Thu, 3 Jun 2021, Peter Xu wrote:

> I found it by pure code review, that pte_same_as_swp() of unuse_vma() didn't

Yes, that is an odd corner, easily missed.

> take uffd-wp bit into account when comparing ptes.  pte_same_as_swp() returning
> false negative could cause failure to swapoff swap ptes that was wr-protected
> by userfaultfd.
> 
> Cc: Hugh Dickins <hughd@...gle.com>
> Cc: Andrea Arcangeli <aarcange@...hat.com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Signed-off-by: Peter Xu <peterx@...hat.com>

I expect you're right: swapoff used to hang forever (but interruptibly)
when this went wrong on powerpc originally.  I don't know the uffd_wp
(nor the soft_dirty) end of it, but treating uffd_wp and soft_dirty
together looks a very good approach, so I'll venture an

Acked-by: Hugh Dickins <hughd@...gle.com>

But I think it should have a uffd_wp Fixes tag and be Cc stable.

> ---
>  include/linux/swapops.h | 15 +++++++++++----
>  mm/swapfile.c           |  2 +-
>  2 files changed, 12 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/swapops.h b/include/linux/swapops.h
> index af3d2661e41e..d356ab4047f7 100644
> --- a/include/linux/swapops.h
> +++ b/include/linux/swapops.h
> @@ -23,6 +23,16 @@
>  #define SWP_TYPE_SHIFT	(BITS_PER_XA_VALUE - MAX_SWAPFILES_SHIFT)
>  #define SWP_OFFSET_MASK	((1UL << SWP_TYPE_SHIFT) - 1)
>  
> +/* Clear all flags but only keep swp_entry_t related information */
> +static inline pte_t pte_swp_clear_flags(pte_t pte)
> +{
> +	if (pte_swp_soft_dirty(pte))
> +		pte = pte_swp_clear_soft_dirty(pte);
> +	if (pte_swp_uffd_wp(pte))
> +		pte = pte_swp_clear_uffd_wp(pte);
> +	return pte;
> +}
> +
>  /*
>   * Store a type+offset into a swp_entry_t in an arch-independent format
>   */
> @@ -66,10 +76,7 @@ static inline swp_entry_t pte_to_swp_entry(pte_t pte)
>  {
>  	swp_entry_t arch_entry;
>  
> -	if (pte_swp_soft_dirty(pte))
> -		pte = pte_swp_clear_soft_dirty(pte);
> -	if (pte_swp_uffd_wp(pte))
> -		pte = pte_swp_clear_uffd_wp(pte);
> +	pte = pte_swp_clear_flags(pte);
>  	arch_entry = __pte_to_swp_entry(pte);
>  	return swp_entry(__swp_type(arch_entry), __swp_offset(arch_entry));
>  }
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 6212bc033602..1e07d1c776f2 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -1887,7 +1887,7 @@ unsigned int count_swap_pages(int type, int free)
>  
>  static inline int pte_same_as_swp(pte_t pte, pte_t swp_pte)
>  {
> -	return pte_same(pte_swp_clear_soft_dirty(pte), swp_pte);
> +	return pte_same(pte_swp_clear_flags(pte), swp_pte);
>  }
>  
>  /*
> -- 
> 2.31.1