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From:   Alistair Popple <apopple@nvidia.com>
To:     Peter Xu <peterx@redhat.com>
CC:     <linux-kernel@vger.kernel.org>, <linux-mm@kvack.org>,
        Axel Rasmussen <axelrasmussen@google.com>,
        "Kirill A . Shutemov" <kirill@shutemov.name>,
        Hugh Dickins <hughd@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Miaohe Lin <linmiaohe@huawei.com>,
        Mike Rapoport <rppt@linux.vnet.ibm.com>,
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        Mike Kravetz <mike.kravetz@oracle.com>,
        Jason Gunthorpe <jgg@ziepe.ca>,
        Matthew Wilcox <willy@infradead.org>
Subject: Re: [PATCH v3 04/27] mm/userfaultfd: Introduce special pte for unmapped file-backed mem
Date:   Wed, 9 Jun 2021 23:06:32 +1000
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On Friday, 28 May 2021 6:19:04 AM AEST Peter Xu wrote:

[...]

> diff --git a/include/asm-generic/pgtable_uffd.h b/include/asm-generic/pgtable_uffd.h
> index 828966d4c281..95e9811ce9d1 100644
> --- a/include/asm-generic/pgtable_uffd.h
> +++ b/include/asm-generic/pgtable_uffd.h
> @@ -2,6 +2,9 @@
>  #define _ASM_GENERIC_PGTABLE_UFFD_H
>  
>  #ifndef CONFIG_HAVE_ARCH_USERFAULTFD_WP
> +
> +#define  UFFD_WP_SWP_PTE_SPECIAL  __pte(0)
> +
>  static __always_inline int pte_uffd_wp(pte_t pte)
>  {
>  	return 0;
> diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
> index 331d2ccf0bcc..93f932b53a71 100644
> --- a/include/linux/userfaultfd_k.h
> +++ b/include/linux/userfaultfd_k.h
> @@ -145,6 +145,17 @@ extern int userfaultfd_unmap_prep(struct vm_area_struct *vma,
>  extern void userfaultfd_unmap_complete(struct mm_struct *mm,
>  				       struct list_head *uf);
>  
> +static inline pte_t pte_swp_mkuffd_wp_special(struct vm_area_struct *vma)
> +{
> +	WARN_ON_ONCE(vma_is_anonymous(vma));
> +	return UFFD_WP_SWP_PTE_SPECIAL;
> +}
> +
> +static inline bool pte_swp_uffd_wp_special(pte_t pte)
> +{
> +	return pte_same(pte, UFFD_WP_SWP_PTE_SPECIAL);
> +}
> +

Sorry, only just noticed this but do we need to define a different version of
this helper that returns false for CONFIG_HAVE_ARCH_USERFAULTFD_WP=n to avoid
spurious matches with __pte(0) on architectures supporting userfaultfd but not
userfaultfd-wp?

>  #else /* CONFIG_USERFAULTFD */
>  
>  /* mm helpers */
> @@ -234,6 +245,16 @@ static inline void userfaultfd_unmap_complete(struct mm_struct *mm,
>  {
>  }
>  
> +static inline pte_t pte_swp_mkuffd_wp_special(struct vm_area_struct *vma)
> +{
> +	return __pte(0);
> +}
> +
> +static inline bool pte_swp_uffd_wp_special(pte_t pte)
> +{
> +	return false;
> +}
> +
>  #endif /* CONFIG_USERFAULTFD */
>  
>  #endif /* _LINUX_USERFAULTFD_K_H */
>