Re: [PATCH v5 09/18] mm: add MM_CP_UFFD_RWP change_protection() flag

[SeongJae Park]

On Tue, 26 May 2026 14:04:57 +0100 Kiryl Shutsemau <kirill@xxxxxxxxxxxxx> wrote:

> From: "Kiryl Shutsemau (Meta)" <kas@xxxxxxxxxx>
> 
> Preparatory patch. Add the change_protection() primitive that
> userfaultfd RWP will use.
> 
> An RWP-protected PTE is PAGE_NONE with the uffd PTE bit set. The
> PROT_NONE half makes the CPU fault on any access; the uffd bit
> distinguishes an RWP fault from a plain mprotect(PROT_NONE) or NUMA
> hinting fault. MM_CP_UFFD_WP and MM_CP_UFFD_RWP share the same PTE
> bit, so the two cannot be used together on the same range.
> 
> Two new change_protection() flags:
> 
>   MM_CP_UFFD_RWP            install PAGE_NONE and set the uffd bit
>   MM_CP_UFFD_RWP_RESOLVE    restore vma->vm_page_prot, clear the uffd bit
> 
> Both are wired through change_pte_range(), change_huge_pmd(), and
> hugetlb_change_protection() so anon, shmem, THP, and hugetlb all
> share the same semantics.
> 
> Signed-off-by: Kiryl Shutsemau <kas@xxxxxxxxxx>
> Assisted-by: Claude:claude-opus-4-6
> Reviewed-by: Mike Rapoport (Microsoft) <rppt@xxxxxxxxxx>

Reviewed-by: SeongJae Park <sj@xxxxxxxxxx>

> ---
>  include/linux/mm.h            |  5 ++++
>  include/linux/userfaultfd_k.h |  1 -
>  mm/huge_memory.c              | 30 +++++++++++++----------
>  mm/hugetlb.c                  | 25 ++++++++++++++-----
>  mm/mprotect.c                 | 46 +++++++++++++++++++++++++++--------
>  5 files changed, 77 insertions(+), 30 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 6499cfb61dc4..f79801816f32 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -3297,6 +3297,11 @@ int get_cmdline(struct task_struct *task, char *buffer, int buflen);
>  #define  MM_CP_UFFD_WP_RESOLVE             (1UL << 3) /* Resolve wp */
>  #define  MM_CP_UFFD_WP_ALL                 (MM_CP_UFFD_WP | \
>  					    MM_CP_UFFD_WP_RESOLVE)
> +/* Whether this change is for uffd RWP */
> +#define  MM_CP_UFFD_RWP                    (1UL << 4) /* do rwp */
> +#define  MM_CP_UFFD_RWP_RESOLVE            (1UL << 5) /* resolve rwp */
> +#define  MM_CP_UFFD_RWP_ALL                (MM_CP_UFFD_RWP | \
> +					    MM_CP_UFFD_RWP_RESOLVE)
>  
>  bool can_change_pte_writable(struct vm_area_struct *vma, unsigned long addr,
>  			     pte_t pte);
> diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
> index 0aef628514df..564eb2aac321 100644
> --- a/include/linux/userfaultfd_k.h
> +++ b/include/linux/userfaultfd_k.h
> @@ -361,7 +361,6 @@ static inline bool userfaultfd_huge_pmd_wp(struct vm_area_struct *vma,
>  	return false;
>  }
>  
> -

Nice cleanup.


Thanks,
SJ

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