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Subject: Re: [PATCH 11/15] mm/swap: add helper swap_offset_available()
To:     NeilBrown <neilb@suse.de>
CC:     <akpm@linux-foundation.org>, <willy@infradead.org>,
        <vbabka@suse.cz>, <dhowells@redhat.com>, <apopple@nvidia.com>,
        <david@redhat.com>, <surenb@google.com>, <peterx@redhat.com>,
        <naoya.horiguchi@nec.com>, <linux-mm@kvack.org>,
        <linux-kernel@vger.kernel.org>
References: <20220509131416.17553-1-linmiaohe@huawei.com>
 <20220509131416.17553-12-linmiaohe@huawei.com>
 <165214355418.14782.13896859043718755300@noble.neil.brown.name>
From:   Miaohe Lin <linmiaohe@huawei.com>
Message-ID: <9319a62b-f43d-8ee9-77b9-a1afee7dbc10@huawei.com>
Date:   Tue, 10 May 2022 10:03:19 +0800
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On 2022/5/10 8:45, NeilBrown wrote:
> 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()

Yes, swap_offset_available_and_locked should be more suitable as we do the spin_lock
inside it. Will do this in next version.

Thanks!

> 
> 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@huawei.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
>>
>>
> .
>