Re: [PATCH v2 09/28] gup: Turn hpage_pincount_add() into page_pincount_add()

[John Hubbard <jhubbard@nvidia.com>]

On 1/9/22 20:23, Matthew Wilcox (Oracle) wrote:
> Simplify try_grab_compound_head() and remove an unnecessary
> VM_BUG_ON by handling pages both with and without a pincount field in
> page_pincount_add().
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> ---
>   mm/gup.c | 33 +++++++++++++++------------------
>   1 file changed, 15 insertions(+), 18 deletions(-)

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


thanks,
-- 
John Hubbard
NVIDIA

> 
> diff --git a/mm/gup.c b/mm/gup.c
> index dbb1b54d0def..3ed9907f3c8d 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -29,12 +29,23 @@ struct follow_page_context {
>   	unsigned int page_mask;
>   };
>   
> -static void hpage_pincount_add(struct page *page, int refs)
> +/*
> + * When pinning a compound page of order > 1 (which is what
> + * hpage_pincount_available() checks for), use an exact count to track
> + * it, via page_pincount_add/_sub().
> + *
> + * However, be sure to *also* increment the normal page refcount field
> + * at least once, so that the page really is pinned.  That's why the
> + * refcount from the earlier try_get_compound_head() is left intact.
> + */
> +static void page_pincount_add(struct page *page, int refs)
>   {
> -	VM_BUG_ON_PAGE(!hpage_pincount_available(page), page);
>   	VM_BUG_ON_PAGE(page != compound_head(page), page);
>   
> -	atomic_add(refs, compound_pincount_ptr(page));
> +	if (hpage_pincount_available(page))
> +		atomic_add(refs, compound_pincount_ptr(page));
> +	else
> +		page_ref_add(page, refs * (GUP_PIN_COUNTING_BIAS - 1));
>   }
>   
>   static void hpage_pincount_sub(struct page *page, int refs)
> @@ -150,21 +161,7 @@ struct page *try_grab_compound_head(struct page *page,
>   		if (!page)
>   			return NULL;
>   
> -		/*
> -		 * When pinning a compound page of order > 1 (which is what
> -		 * hpage_pincount_available() checks for), use an exact count to
> -		 * track it, via hpage_pincount_add/_sub().
> -		 *
> -		 * However, be sure to *also* increment the normal page refcount
> -		 * field at least once, so that the page really is pinned.
> -		 * That's why the refcount from the earlier
> -		 * try_get_compound_head() is left intact.
> -		 */
> -		if (hpage_pincount_available(page))
> -			hpage_pincount_add(page, refs);
> -		else
> -			page_ref_add(page, refs * (GUP_PIN_COUNTING_BIAS - 1));
> -
> +		page_pincount_add(page, refs);
>   		mod_node_page_state(page_pgdat(page), NR_FOLL_PIN_ACQUIRED,
>   				    refs);
>