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Date:   Sat, 5 Feb 2022 18:12:18 -0800
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Subject: Re: [PATCH 16/75] mm/gup: Convert try_grab_page() to use a folio
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To:     "Matthew Wilcox (Oracle)" <willy@infradead.org>, linux-mm@kvack.org
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From:   John Hubbard <jhubbard@nvidia.com>
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On 2/4/22 11:57, Matthew Wilcox (Oracle) wrote:
> Hoist the folio conversion and the folio_ref_count() check to the
> top of the function instead of using the one buried in try_get_page().
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> ---
>   mm/gup.c | 28 +++++++++++++---------------
>   1 file changed, 13 insertions(+), 15 deletions(-)

Reviewed-by: John Hubbard <jhubbard@nvidia.com>

thanks,
-- 
John Hubbard
NVIDIA

> 
> diff --git a/mm/gup.c b/mm/gup.c
> index 4f1669db92f5..d18ce4da573f 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -174,15 +174,14 @@ static void put_compound_head(struct page *page, int refs, unsigned int flags)
>   
>   /**
>    * try_grab_page() - elevate a page's refcount by a flag-dependent amount
> + * @page:    pointer to page to be grabbed
> + * @flags:   gup flags: these are the FOLL_* flag values.
>    *
>    * This might not do anything at all, depending on the flags argument.
>    *
>    * "grab" names in this file mean, "look at flags to decide whether to use
>    * FOLL_PIN or FOLL_GET behavior, when incrementing the page's refcount.
>    *
> - * @page:    pointer to page to be grabbed
> - * @flags:   gup flags: these are the FOLL_* flag values.
> - *
>    * Either FOLL_PIN or FOLL_GET (or neither) may be set, but not both at the same
>    * time. Cases: please see the try_grab_folio() documentation, with
>    * "refs=1".
> @@ -193,29 +192,28 @@ static void put_compound_head(struct page *page, int refs, unsigned int flags)
>    */
>   bool __must_check try_grab_page(struct page *page, unsigned int flags)
>   {
> +	struct folio *folio = page_folio(page);
> +
>   	WARN_ON_ONCE((flags & (FOLL_GET | FOLL_PIN)) == (FOLL_GET | FOLL_PIN));
> +	if (WARN_ON_ONCE(folio_ref_count(folio) <= 0))
> +		return false;
>   
>   	if (flags & FOLL_GET)
> -		return try_get_page(page);
> +		folio_ref_inc(folio);
>   	else if (flags & FOLL_PIN) {
> -		page = compound_head(page);
> -
> -		if (WARN_ON_ONCE(page_ref_count(page) <= 0))
> -			return false;
> -
>   		/*
> -		 * Similar to try_grab_compound_head(): be sure to *also*
> +		 * Similar to try_grab_folio(): be sure to *also*
>   		 * increment the normal page refcount field at least once,
>   		 * so that the page really is pinned.
>   		 */
> -		if (PageHead(page)) {
> -			page_ref_add(page, 1);
> -			atomic_add(1, compound_pincount_ptr(page));
> +		if (folio_test_large(folio)) {
> +			folio_ref_add(folio, 1);
> +			atomic_add(1, folio_pincount_ptr(folio));
>   		} else {
> -			page_ref_add(page, GUP_PIN_COUNTING_BIAS);
> +			folio_ref_add(folio, GUP_PIN_COUNTING_BIAS);
>   		}
>   
> -		mod_node_page_state(page_pgdat(page), NR_FOLL_PIN_ACQUIRED, 1);
> +		node_stat_mod_folio(folio, NR_FOLL_PIN_ACQUIRED, 1);
>   	}
>   
>   	return true;