linux-kernel - Re: [PATCH 11/15] mm/swap: add helper swap_offset

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Date:   Tue, 10 May 2022 10:45:54 +1000
From:   "NeilBrown" <neilb@...e.de>
To:     "Miaohe Lin" <linmiaohe@...wei.com>
Cc:     akpm@...ux-foundation.org, willy@...radead.org, vbabka@...e.cz,
        dhowells@...hat.com, apopple@...dia.com, david@...hat.com,
        surenb@...gle.com, peterx@...hat.com, naoya.horiguchi@....com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linmiaohe@...wei.com
Subject: Re: [PATCH 11/15] mm/swap: add helper swap_offset_available()

On Mon, 09 May 2022, Miaohe Lin wrote:
> Add helper swap_offset_available() to remove some duplicated codes.
> Minor readability improvement.

I don't think that putting the spin_lock() inside the inline helper is
good for readability.
If the function was called
   swap_offset_available_and_locked()

it might be ok.  Otherwise I would rather the spin_lock() was called
when the function returned true.

Thanks,
NeilBrown

> 
> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
> ---
>  mm/swapfile.c | 33 +++++++++++++++++----------------
>  1 file changed, 17 insertions(+), 16 deletions(-)
> 
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index c90298a0561a..d5d3e2d03d28 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -776,6 +776,21 @@ static void set_cluster_next(struct swap_info_struct *si, unsigned long next)
>  	this_cpu_write(*si->cluster_next_cpu, next);
>  }
>  
> +static inline bool swap_offset_available(struct swap_info_struct *si, unsigned long offset)
> +{
> +	if (data_race(!si->swap_map[offset])) {
> +		spin_lock(&si->lock);
> +		return true;
> +	}
> +
> +	if (vm_swap_full() && READ_ONCE(si->swap_map[offset]) == SWAP_HAS_CACHE) {
> +		spin_lock(&si->lock);
> +		return true;
> +	}
> +
> +	return false;
> +}
> +
>  static int scan_swap_map_slots(struct swap_info_struct *si,
>  			       unsigned char usage, int nr,
>  			       swp_entry_t slots[])
> @@ -953,15 +968,8 @@ static int scan_swap_map_slots(struct swap_info_struct *si,
>  scan:
>  	spin_unlock(&si->lock);
>  	while (++offset <= READ_ONCE(si->highest_bit)) {
> -		if (data_race(!si->swap_map[offset])) {
> -			spin_lock(&si->lock);
> +		if (swap_offset_available(si, offset))
>  			goto checks;
> -		}
> -		if (vm_swap_full() &&
> -		    READ_ONCE(si->swap_map[offset]) == SWAP_HAS_CACHE) {
> -			spin_lock(&si->lock);
> -			goto checks;
> -		}
>  		if (unlikely(--latency_ration < 0)) {
>  			cond_resched();
>  			latency_ration = LATENCY_LIMIT;
> @@ -970,15 +978,8 @@ static int scan_swap_map_slots(struct swap_info_struct *si,
>  	}
>  	offset = si->lowest_bit;
>  	while (offset < scan_base) {
> -		if (data_race(!si->swap_map[offset])) {
> -			spin_lock(&si->lock);
> +		if (swap_offset_available(si, offset))
>  			goto checks;
> -		}
> -		if (vm_swap_full() &&
> -		    READ_ONCE(si->swap_map[offset]) == SWAP_HAS_CACHE) {
> -			spin_lock(&si->lock);
> -			goto checks;
> -		}
>  		if (unlikely(--latency_ration < 0)) {
>  			cond_resched();
>  			latency_ration = LATENCY_LIMIT;
> -- 
> 2.23.0
> 
>