linux-kernel - Re: [PATCH] mm/mremap_pages: Save a few cycles in 'get_dev

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Message-ID: <6754c5ec-7b89-0393-659b-5c0487a70138@wanadoo.fr>
Date:   Fri, 12 Aug 2022 06:58:45 +0200
From:   Christophe JAILLET <christophe.jaillet@...adoo.fr>
To:     akpm@...ux-foundation.org
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        kernel-janitors@...r.kernel.org
Subject: Re: [PATCH] mm/mremap_pages: Save a few cycles in 'get_dev_pagemap()'

Le 03/11/2021 à 22:35, Christophe JAILLET a écrit :
> Use 'percpu_ref_tryget_live_rcu()' instead of 'percpu_ref_tryget_live()' to
> save a few cycles when it is known that the rcu lock is already
> taken/released.
> 
> Signed-off-by: Christophe JAILLET <christophe.jaillet@...adoo.fr>
> ---
>   mm/memremap.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/memremap.c b/mm/memremap.c
> index 84de22c14567..012e8d23d365 100644
> --- a/mm/memremap.c
> +++ b/mm/memremap.c
> @@ -506,7 +506,7 @@ struct dev_pagemap *get_dev_pagemap(unsigned long pfn,
>   	/* fall back to slow path lookup */
>   	rcu_read_lock();
>   	pgmap = xa_load(&pgmap_array, PHYS_PFN(phys));
> -	if (pgmap && !percpu_ref_tryget_live(pgmap->ref))
> +	if (pgmap && !percpu_ref_tryget_live_rcu(pgmap->ref))
>   		pgmap = NULL;
>   	rcu_read_unlock();
>   

Hi,
gentle reminder.

Is this patch useful?
When I first posted it, percpu_ref_tryget_live_rcu() was really new.
Now it is part of linux since 5.16.

CJ