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Subject: Re: [PATCH 2/2] mm: avoid unnecessary swap_info_struct allocation
To:     "Huang, Ying" <ying.huang@intel.com>
Cc:     linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
        linux-kernel@vger.kernel.org, Aaron Lu <aaron.lu@intel.com>
References: <a24bf353-8715-2bee-d0fa-96ca06c5b69f@virtuozzo.com>
 <87sh0gbau6.fsf@yhuang-dev.intel.com>
From:   Vasily Averin <vvs@virtuozzo.com>
Message-ID: <bf132d36-e9cd-8c27-fffa-f3e734065aec@virtuozzo.com>
Date:   Mon, 5 Nov 2018 08:19:27 +0300
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On 11/5/18 3:57 AM, Huang, Ying wrote:
> Vasily Averin <vvs@virtuozzo.com> writes:
> 
>> Currently newly allocated swap_info_struct can be quickly freed.
>> This patch avoid uneccessary high-order page allocation and helps
>> to decrease the memory pressure.
> 
> I think swapon/swapoff are rare operations, so it will not increase the
> memory pressure much.

You are right, typically it should not affect usual nodes.

It's OpenVz-specific usecase.

OpenVz allows hosters to run hundreds of non-trusted containers per node.
Our containers have enabled "virtual swap" functionality, 
and container's owners can call sys_swapon without any limits.
Containers can be restarted in any time and we would like to 
decrease number of unnecessary high-order memory allocations.

>> Signed-off-by: Vasily Averin <vvs@virtuozzo.com>
>> ---
>>  mm/swapfile.c | 18 +++++++++++++-----
>>  1 file changed, 13 insertions(+), 5 deletions(-)
>>
>> diff --git a/mm/swapfile.c b/mm/swapfile.c
>> index 8688ae65ef58..53ec2f0cdf26 100644
>> --- a/mm/swapfile.c
>> +++ b/mm/swapfile.c
>> @@ -2809,14 +2809,17 @@ late_initcall(max_swapfiles_check);
>>  
>>  static struct swap_info_struct *alloc_swap_info(void)
>>  {
>> -	struct swap_info_struct *p;
>> +	struct swap_info_struct *p = NULL;
>>  	unsigned int type;
>>  	int i;
>> +	bool force_alloc = false;
>>  
>> -	p = kvzalloc(sizeof(*p), GFP_KERNEL);
>> -	if (!p)
>> -		return ERR_PTR(-ENOMEM);
>> -
>> +retry:
>> +	if (force_alloc) {
>> +		p = kvzalloc(sizeof(*p), GFP_KERNEL);
>> +		if (!p)
>> +			return ERR_PTR(-ENOMEM);
>> +	}
>>  	spin_lock(&swap_lock);
>>  	for (type = 0; type < nr_swapfiles; type++) {
>>  		if (!(swap_info[type]->flags & SWP_USED))
>> @@ -2828,6 +2831,11 @@ static struct swap_info_struct *alloc_swap_info(void)
>>  		return ERR_PTR(-EPERM);
>>  	}
>>  	if (type >= nr_swapfiles) {
>> +		if (!force_alloc) {
>> +			force_alloc = true;
>> +			spin_unlock(&swap_lock);
>> +			goto retry;
>> +		}
>>  		p->type = type;
>>  		swap_info[type] = p;
>>  		/*
>