Re: [PATCH 01/75] mm/gup: Increment the page refcount before the pincount

[John Hubbard]

On 2/4/22 11:57, Matthew Wilcox (Oracle) wrote:
> We should always increase the refcount before doing anything else to
> the page so that other page users see the elevated refcount first.

Absolutely agree in principle. Is there anything else to say, though,
such as why this matters here? Or is the change just being done for
"best practices"? (Which is still a very solid reason, of course.)
> Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> ---
>   mm/gup.c | 12 ++++++------
>   1 file changed, 6 insertions(+), 6 deletions(-)
>
> diff --git a/mm/gup.c b/mm/gup.c
> index a9d4d724aef7..08020987dfc0 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -220,18 +220,18 @@ bool __must_check try_grab_page(struct page *page, unsigned int flags)
>   		if (WARN_ON_ONCE(page_ref_count(page) <= 0))
>   			return false;
>   
> -		if (hpage_pincount_available(page))
> -			hpage_pincount_add(page, 1);
> -		else
> -			refs = GUP_PIN_COUNTING_BIAS;
> -
>   		/*
>   		 * Similar to try_grab_compound_head(): even if using the
>   		 * hpage_pincount_add/_sub() routines, be sure to
>   		 * *also* increment the normal page refcount field at least
>   		 * once, so that the page really is pinned.
>   		 */
> -		page_ref_add(page, refs);

A fine point: this hunk removes the last use of "refs", which means that
this patch will lead to an unused variable warning. So I think it would
be best to remove the "int refs = 1;" line in this patch, rather than
waiting until patch #10.

With that change, please feel free to add:

Reviewed-by: John Hubbard <jhubbard@xxxxxxxxxx>


thanks,
--
John Hubbard
NVIDIA

> +		if (hpage_pincount_available(page)) {
> +			page_ref_add(page, 1);
> +			hpage_pincount_add(page, 1);
> +		} else {
> +			page_ref_add(page, GUP_PIN_COUNTING_BIAS);
> +		}
>   
>   		mod_node_page_state(page_pgdat(page), NR_FOLL_PIN_ACQUIRED, 1);
>   	}