Re: [PATCH v3 04/16] mm/rmap: split page_dup_rmap() into page_dup_file_rmap() and page_try_dup_anon_rmap()

[Vlastimil Babka]

On 3/29/22 18:04, David Hildenbrand wrote:
> ... and move the special check for pinned pages into
> page_try_dup_anon_rmap() to prepare for tracking exclusive anonymous
> pages via a new pageflag, clearing it only after making sure that there
> are no GUP pins on the anonymous page.
> 
> We really only care about pins on anonymous pages, because they are
> prone to getting replaced in the COW handler once mapped R/O. For !anon
> pages in cow-mappings (!VM_SHARED && VM_MAYWRITE) we shouldn't really
> care about that, at least not that I could come up with an example.
> 
> Let's drop the is_cow_mapping() check from page_needs_cow_for_dma(), as we
> know we're dealing with anonymous pages. Also, drop the handling of
> pinned pages from copy_huge_pud() and add a comment if ever supporting
> anonymous pages on the PUD level.
> 
> This is a preparation for tracking exclusivity of anonymous pages in
> the rmap code, and disallowing marking a page shared (-> failing to
> duplicate) if there are GUP pins on a page.
> 
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

Nit:

> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -825,7 +825,8 @@ copy_nonpresent_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm,
>  		 */
>  		get_page(page);
>  		rss[mm_counter(page)]++;
> -		page_dup_rmap(page, false);
> +		/* Cannot fail as these pages cannot get pinned. */
> +		BUG_ON(page_try_dup_anon_rmap(page, false, src_vma));

Should we just call __page_dup_rmap() here? This is block for the condition
is_device_private_entry(), and page_try_dup_anon_rmap() can't return -EBUSY
for is_device_private_page().

>  
>  		/*
>  		 * We do not preserve soft-dirty information, because so
> @@ -921,18 +922,24 @@ 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 && unlikely(page_needs_cow_for_dma(src_vma, page))) {
> +	if (page && PageAnon(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);
> +		get_page(page);
> +		if (unlikely(page_try_dup_anon_rmap(page, false, src_vma))) {
> +			/* Page maybe pinned, we have to copy. */
> +			put_page(page);
> +			return copy_present_page(dst_vma, src_vma, dst_pte, src_pte,
> +						 addr, rss, prealloc, page);
> +		}
> +		rss[mm_counter(page)]++;
>  	} else if (page) {
>  		get_page(page);
> -		page_dup_rmap(page, false);
> +		page_dup_file_rmap(page, false);
>  		rss[mm_counter(page)]++;
>  	}
>  
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 3d60823afd2d..97de2fc17f34 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -234,7 +234,7 @@ static bool remove_migration_pte(struct folio *folio,
>  			if (folio_test_anon(folio))
>  				hugepage_add_anon_rmap(new, vma, pvmw.address);
>  			else
> -				page_dup_rmap(new, true);
> +				page_dup_file_rmap(new, true);
>  			set_huge_pte_at(vma->vm_mm, pvmw.address, pvmw.pte, pte);
>  		} else
>  #endif