Re: [RFC PATCH 01/14] userfaultfd: set dirty and young on writeprotect

[David Hildenbrand]

On 18.07.22 14:01, Nadav Amit wrote:
> From: Nadav Amit <namit@xxxxxxxxxx>
> 
> When userfaultfd makes a PTE writable, it can now change the PTE
> directly, in some cases, without going triggering a page-fault first.
> Yet, doing so might leave the PTE that was write-unprotected as old and
> clean. At least on x86, this would cause a >500 cycles overhead when the
> PTE is first accessed.
> 
> Use MM_CP_WILL_NEED to set the PTE as young and dirty when userfaultfd
> gets a hint that the page is likely to be used. Avoid changing the PTE
> to young and dirty in other cases to avoid excessive writeback and
> messing with the page reclamation logic.
> 
> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Andy Lutomirski <luto@xxxxxxxxxx>
> Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> Cc: David Hildenbrand <david@xxxxxxxxxx>
> Cc: Peter Xu <peterx@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Will Deacon <will@xxxxxxxxxx>
> Cc: Yu Zhao <yuzhao@xxxxxxxxxx>
> Cc: Nick Piggin <npiggin@xxxxxxxxx>
> ---
>  include/linux/mm.h | 2 ++
>  mm/mprotect.c      | 9 ++++++++-
>  mm/userfaultfd.c   | 8 ++++++--
>  3 files changed, 16 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 9cc02a7e503b..4afd75ce5875 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1988,6 +1988,8 @@ extern unsigned long move_page_tables(struct vm_area_struct *vma,
>  /* Whether this change is for write protecting */
>  #define  MM_CP_UFFD_WP                     (1UL << 2) /* do wp */
>  #define  MM_CP_UFFD_WP_RESOLVE             (1UL << 3) /* Resolve wp */
> +/* Whether to try to mark entries as dirty as they are to be written */
> +#define  MM_CP_WILL_NEED		   (1UL << 4)
>  #define  MM_CP_UFFD_WP_ALL                 (MM_CP_UFFD_WP | \
>  					    MM_CP_UFFD_WP_RESOLVE)
>  
> diff --git a/mm/mprotect.c b/mm/mprotect.c
> index 996a97e213ad..34c2dfb68c42 100644
> --- a/mm/mprotect.c
> +++ b/mm/mprotect.c
> @@ -82,6 +82,7 @@ static unsigned long change_pte_range(struct mmu_gather *tlb,
>  	bool prot_numa = cp_flags & MM_CP_PROT_NUMA;
>  	bool uffd_wp = cp_flags & MM_CP_UFFD_WP;
>  	bool uffd_wp_resolve = cp_flags & MM_CP_UFFD_WP_RESOLVE;
> +	bool will_need = cp_flags & MM_CP_WILL_NEED;
>  
>  	tlb_change_page_size(tlb, PAGE_SIZE);
>  
> @@ -172,6 +173,9 @@ static unsigned long change_pte_range(struct mmu_gather *tlb,
>  				ptent = pte_clear_uffd_wp(ptent);
>  			}
>  
> +			if (will_need)
> +				ptent = pte_mkyoung(ptent);
> +
>  			/*
>  			 * In some writable, shared mappings, we might want
>  			 * to catch actual write access -- see
> @@ -187,8 +191,11 @@ static unsigned long change_pte_range(struct mmu_gather *tlb,
>  			 */
>  			if ((cp_flags & MM_CP_TRY_CHANGE_WRITABLE) &&
>  			    !pte_write(ptent) &&


Why would we want to check if we can set something writable if it
already *is* writable? That doesn't make sense to me.

> -			    can_change_pte_writable(vma, addr, ptent))
> +			    can_change_pte_writable(vma, addr, ptent)) {
>  				ptent = pte_mkwrite(ptent);
> +				if (will_need)
> +					ptent = pte_mkdirty(ptent);
> +			}

-- 
Thanks,

David / dhildenb