Re: [PATCH v2 4/5] userfaultfd: zero access/write hints

[David Hildenbrand <david@redhat.com>]

On 18.07.22 13:47, Nadav Amit wrote:
> From: Nadav Amit <namit@vmware.com>
> 
> When userfaultfd provides a zeropage in response to ioctl, it provides a
> readonly alias to the zero page. If the page is later written (which is
> the likely scenario), page-fault occurs and the page-fault allocator
> allocates a page and rewires the page-tables.
> 
> This is an expensive flow for cases in which a page is likely be written
> to. Users can use the copy ioctl to initialize zero page (by copying
> zeros), but this is also wasteful.
> 
> Allow userfaultfd users to efficiently map initialized zero-pages that
> are writable. IF UFFDIO_ZEROPAGE_MODE_WRITE_LIKELY is provided would map
> a clear page instead of an alias to the zero page.
> 
> Suggested-by: David Hildenbrand <david@redhat.com>
> Cc: Mike Kravetz <mike.kravetz@oracle.com>
> Cc: Hugh Dickins <hughd@google.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Axel Rasmussen <axelrasmussen@google.com>
> Cc: Mike Rapoport <rppt@linux.ibm.com>
> Acked-by: Peter Xu <peterx@redhat.com>
> Signed-off-by: Nadav Amit <namit@vmware.com>
> ---
>  mm/userfaultfd.c | 35 +++++++++++++++++++++++++++++++++++
>  1 file changed, 35 insertions(+)
> 
> diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> index c15679f3eb6a..954c6980b29f 100644
> --- a/mm/userfaultfd.c
> +++ b/mm/userfaultfd.c
> @@ -241,6 +241,37 @@ static int mfill_zeropage_pte(struct mm_struct *dst_mm,
>  	return ret;
>  }
>  
> +static int mfill_clearpage_pte(struct mm_struct *dst_mm, pmd_t *dst_pmd,
> +			       struct vm_area_struct *dst_vma,
> +			       unsigned long dst_addr,
> +			       uffd_flags_t uffd_flags)
> +{
> +	struct page *page;
> +	int ret;
> +
> +	ret = -ENOMEM;
> +	page = alloc_zeroed_user_highpage_movable(dst_vma, dst_addr);
> +	if (!page)
> +		goto out;
> +
> +	/* The PTE is not marked as dirty unconditionally */
> +	SetPageDirty(page);
> +	__SetPageUptodate(page);
> +
> +	if (mem_cgroup_charge(page_folio(page), dst_vma->vm_mm, GFP_KERNEL))
> +		goto out_release;
> +
> +	ret = mfill_atomic_install_pte(dst_mm, dst_pmd, dst_vma, dst_addr,
> +				       page, true, uffd_flags);
> +	if (ret)
> +		goto out_release;
> +out:
> +	return ret;
> +out_release:
> +	put_page(page);
> +	goto out;
> +}
> +
>  /* Handles UFFDIO_CONTINUE for all shmem VMAs (shared or private). */
>  static int mcontinue_atomic_pte(struct mm_struct *dst_mm,
>  				pmd_t *dst_pmd,
> @@ -500,6 +531,10 @@ static __always_inline ssize_t mfill_atomic_pte(struct mm_struct *dst_mm,
>  			err = mcopy_atomic_pte(dst_mm, dst_pmd, dst_vma,
>  					       dst_addr, src_addr, page,
>  					       uffd_flags);
> +		else if (!(uffd_flags & UFFD_FLAGS_WP) &&
> +			 (uffd_flags & UFFD_FLAGS_WRITE_LIKELY))
> +			err = mfill_clearpage_pte(dst_mm, dst_pmd, dst_vma,
> +						  dst_addr, uffd_flags);
>  		else
>  			err = mfill_zeropage_pte(dst_mm, dst_pmd,
>  						 dst_vma, dst_addr, uffd_flags);

Reviewed-by: David Hildenbrand <david@redhat.com>

-- 
Thanks,

David / dhildenb