Re: [PATCH RFC 03/13] mm/memory: slightly simplify copy_present_pte()

[Hillf Danton <hdanton@sina.com>]

On Thu, 24 Feb 2022 13:26:04 +0100 David Hildenbrand wrote:
> Let's move the pinning check into the caller, to simplify return code
> logic and prepare for further changes: relocating the
> page_needs_cow_for_dma() into rmap handling code.
> 
> While at it, remove the unused pte parameter and simplify the comments a
> bit.
> 
> No functional change intended.
> 
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
>  mm/memory.c | 53 ++++++++++++++++-------------------------------------
>  1 file changed, 16 insertions(+), 37 deletions(-)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index c6177d897964..accb72a3343d 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -865,19 +865,11 @@ copy_nonpresent_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm,
>  }
>  
>  /*
> - * Copy a present and normal page if necessary.
> + * Copy a present and normal page.
>   *
> - * NOTE! The usual case is that this doesn't need to do
> - * anything, and can just return a positive value. That
> - * will let the caller know that it can just increase
> - * the page refcount and re-use the pte the traditional
> - * way.
> - *
> - * But _if_ we need to copy it because it needs to be
> - * pinned in the parent (and the child should get its own
> - * copy rather than just a reference to the same page),
> - * we'll do that here and return zero to let the caller
> - * know we're done.
> + * NOTE! The usual case is that this isn't required;
> + * instead, the caller can just increase the page refcount
> + * and re-use the pte the traditional way.
>   *
>   * And if we need a pre-allocated page but don't yet have
>   * one, return a negative error to let the preallocation
> @@ -887,25 +879,10 @@ copy_nonpresent_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm,
>  static inline int
>  copy_present_page(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma,
>  		  pte_t *dst_pte, pte_t *src_pte, unsigned long addr, int *rss,
> -		  struct page **prealloc, pte_t pte, struct page *page)
> +		  struct page **prealloc, struct page *page)
>  {
>  	struct page *new_page;
> -
> -	/*
> -	 * What we want to do is to check whether this page may
> -	 * have been pinned by the parent process.  If so,
> -	 * instead of wrprotect the pte on both sides, we copy
> -	 * the page immediately so that we'll always guarantee
> -	 * the pinned page won't be randomly replaced in the
> -	 * future.
> -	 *
> -	 * The page pinning checks are just "has this mm ever
> -	 * seen pinning", along with the (inexact) check of
> -	 * the page count. That might give false positives for
> -	 * for pinning, but it will work correctly.
> -	 */
> -	if (likely(!page_needs_cow_for_dma(src_vma, page)))
> -		return 1;
> +	pte_t pte;
>  
>  	new_page = *prealloc;
>  	if (!new_page)
> @@ -947,14 +924,16 @@ copy_present_pte(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma,
>  	struct page *page;
>  
>  	page = vm_normal_page(src_vma, addr, pte);
> -	if (page) {
> -		int retval;
> -
> -		retval = copy_present_page(dst_vma, src_vma, dst_pte, src_pte,
> -					   addr, rss, prealloc, pte, page);
> -		if (retval <= 0)
> -			return retval;
> -
> +	if (page && unlikely(page_needs_cow_for_dma(src_vma, page))) {
> +		/*
> +		 * If this page may have been pinned by the parent process,
> +		 * copy the page immediately for the child so that we'll always
> +		 * guarantee the pinned page won't be randomly replaced in the
> +		 * future.
> +		 */
> +		return copy_present_page(dst_vma, src_vma, dst_pte, src_pte,
> +					 addr, rss, prealloc, page);

Off topic question, is it likely that the DMA tranfer from device in parallel
to copying page ends up with different data between parent and kid?

Hillf

> +	} else if (page) {
>  		get_page(page);
>  		page_dup_rmap(page, false);
>  		rss[mm_counter(page)]++;
> -- 
> 2.35.1