netdev - Re: [PATCH net-next v9 2/4] page_pool: halve BIAS_MAX for multiple user references of a fragment

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Date: Wed, 13 Dec 2023 13:38:38 +0200
From: Ilias Apalodimas <ilias.apalodimas@...aro.org>
To: Liang Chen <liangchen.linux@...il.com>
Cc: davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org, 
	pabeni@...hat.com, hawk@...nel.org, linyunsheng@...wei.com, 
	netdev@...r.kernel.org, linux-mm@...ck.org, jasowang@...hat.com, 
	almasrymina@...gle.com
Subject: Re: [PATCH net-next v9 2/4] page_pool: halve BIAS_MAX for multiple
 user references of a fragment

Hi Liang,

On Tue, 12 Dec 2023 at 06:47, Liang Chen <liangchen.linux@...il.com> wrote:
>
> Referring to patch [1], in order to support multiple users referencing the
> same fragment and prevent overflow from pp_ref_count growing, the initial
> value of pp_ref_count is halved, leaving room for pp_ref_count to increment
> before the page is drained.
>
> [1]
> https://lore.kernel.org/all/20211009093724.10539-3-linyunsheng@huawei.com/

We only need this if patch #4 is merged.  In that case, I'd like to
describe the changelog a bit better.
Something along the lines of
"Up to now, we were only subtracting from the number of used page
fragments to figure out when a page could be freed or recycled. A
following patch introduces support for multiple users referencing the
same fragment. So reduce the initial page fragments value to half to
avoid overflowing"

Thanks
/Ilias
> same fragment



>
> Signed-off-by: Liang Chen <liangchen.linux@...il.com>
> Reviewed-by: Yunsheng Lin <linyunsheng@...wei.com>
> ---
>  net/core/page_pool.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/net/core/page_pool.c b/net/core/page_pool.c
> index 106220b1f89c..436f7ffea7b4 100644
> --- a/net/core/page_pool.c
> +++ b/net/core/page_pool.c
> @@ -26,7 +26,7 @@
>  #define DEFER_TIME (msecs_to_jiffies(1000))
>  #define DEFER_WARN_INTERVAL (60 * HZ)
>
> -#define BIAS_MAX       LONG_MAX
> +#define BIAS_MAX       (LONG_MAX >> 1)
>
>  #ifdef CONFIG_PAGE_POOL_STATS
>  /* alloc_stat_inc is intended to be used in softirq context */
> --
> 2.31.1
>