linux-kernel - Re: [PATCH] mm/swap_slots.c: Remove unnecessary NULL pointer check

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Date:   Thu, 14 Jan 2021 10:34:57 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     Matthew Wilcox <willy@...radead.org>
CC:     <akpm@...ux-foundation.org>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm/swap_slots.c: Remove unnecessary NULL pointer check

Hi:
On 2021/1/10 1:40, Matthew Wilcox wrote:
> On Sat, Jan 09, 2021 at 03:09:43AM -0500, Miaohe Lin wrote:
>> The cache->slots and cache->slots_ret is already checked before we try to
>> drain it. And kvfree can handle the NULL pointer itself. So remove the
>> NULL pointer check here.
> 
>> @@ -178,7 +178,7 @@ static void drain_slots_cache_cpu(unsigned int cpu, unsigned int type,
>>  		swapcache_free_entries(cache->slots + cache->cur, cache->nr);
>>  		cache->cur = 0;
>>  		cache->nr = 0;
>> -		if (free_slots && cache->slots) {
>> +		if (free_slots) {
> 
> Prove that swapcache_free_entries() doesn't change cache->slots.
> 

Yeh... I see. I thought swap_slots_cache_mutex could totally guard against this.

>> @@ -188,13 +188,12 @@ static void drain_slots_cache_cpu(unsigned int cpu, unsigned int type,
>>  		spin_lock_irq(&cache->free_lock);
>>  		swapcache_free_entries(cache->slots_ret, cache->n_ret);
>>  		cache->n_ret = 0;
>> -		if (free_slots && cache->slots_ret) {
>> +		if (free_slots) {
> 
> ... or ->slots_ret
> 
>> -		if (slots)
>> -			kvfree(slots);
>> +		kvfree(slots);
> 
> This is fine.
> .
> 

Many thanks for your review and reply!