linux-kernel - Re: [PATCH v1 18/36] mm/gup: drop nth_page() usage within folio when recording subpages

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Message-ID: <c0dadc4f-6415-4818-a319-e3e15ff47a24@lucifer.local>
Date: Thu, 28 Aug 2025 17:37:54 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: David Hildenbrand <david@...hat.com>
Cc: linux-kernel@...r.kernel.org, Alexander Potapenko <glider@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Brendan Jackman <jackmanb@...gle.com>,
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Subject: Re: [PATCH v1 18/36] mm/gup: drop nth_page() usage within folio when
 recording subpages

On Thu, Aug 28, 2025 at 12:01:22AM +0200, David Hildenbrand wrote:
> nth_page() is no longer required when iterating over pages within a
> single folio, so let's just drop it when recording subpages.
>
> Signed-off-by: David Hildenbrand <david@...hat.com>

This looks correct to me, so notwithtsanding suggestion below, LGTM and:

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>

> ---
>  mm/gup.c | 7 +++----
>  1 file changed, 3 insertions(+), 4 deletions(-)
>
> diff --git a/mm/gup.c b/mm/gup.c
> index b2a78f0291273..89ca0813791ab 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -488,12 +488,11 @@ static int record_subpages(struct page *page, unsigned long sz,
>  			   unsigned long addr, unsigned long end,
>  			   struct page **pages)
>  {
> -	struct page *start_page;
>  	int nr;
>
> -	start_page = nth_page(page, (addr & (sz - 1)) >> PAGE_SHIFT);
> +	page += (addr & (sz - 1)) >> PAGE_SHIFT;
>  	for (nr = 0; addr != end; nr++, addr += PAGE_SIZE)
> -		pages[nr] = nth_page(start_page, nr);
> +		pages[nr] = page++;


This is really nice, but I wonder if (while we're here) we can't be even
more clear as to what's going on here, e.g.:

static int record_subpages(struct page *page, unsigned long sz,
			   unsigned long addr, unsigned long end,
			   struct page **pages)
{
	size_t offset_in_folio = (addr & (sz - 1)) >> PAGE_SHIFT;
	struct page *subpage = page + offset_in_folio;

	for (; addr != end; addr += PAGE_SIZE)
		*pages++ = subpage++;

	return nr;
}

Or some variant of that with the masking stuff self-documented.

>
>  	return nr;
>  }
> @@ -1512,7 +1511,7 @@ static long __get_user_pages(struct mm_struct *mm,
>  			}
>
>  			for (j = 0; j < page_increm; j++) {
> -				subpage = nth_page(page, j);
> +				subpage = page + j;
>  				pages[i + j] = subpage;
>  				flush_anon_page(vma, subpage, start + j * PAGE_SIZE);
>  				flush_dcache_page(subpage);
> --
> 2.50.1
>

Cheers, Lorenzo