Re: [RFC PATCH] swapfile: fix soft lockup in scan_swap_map_slots

[Huang, Ying]

Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> writes:

> On Fri, 18 Nov 2022 21:38:50 +0800 Chen Wandun <chenwandun@xxxxxxxxxx> wrote:
>
>> A soft lockup occur in scan free swap slot by constructing
>> huge memory pressure.
>> The test scenario is: 64 CPU cores, 64GB memory, and 28
>> zram devices, the disksize of each zram device is 50MB.
>> 
>> LATENCY_LIMIT is used to prevent soft lockup in function
>> scan_swap_map_slots, but the real loop number would more
>> than LATENCY_LIMIT because of "goto checks and goto scan"
>> repeatly without decrease of latency limit.
>> 
>> In order to fix it, move decrease latency_ration code in advance.
>> 
>> There is also a suspicious place that will cause soft lockup in
>> function get_swap_pages, in this function, the "goto start_over"
>> may result in continuous scanning of swap partition, if there is
>> no cond_sched in scan_swap_map_slots, it would cause soft lockup
>> (I am not sure about this).
>> 
>> ...
>>
>
> Looks sensible.

Yes.  LGTM.

Reviewed-by: "Huang, Ying" <ying.huang@xxxxxxxxx>

>> --- a/mm/swapfile.c
>> +++ b/mm/swapfile.c
>> @@ -972,23 +972,23 @@ static int scan_swap_map_slots(struct swap_info_struct *si,
>>  scan:
>>  	spin_unlock(&si->lock);
>>  	while (++offset <= READ_ONCE(si->highest_bit)) {
>> -		if (swap_offset_available_and_locked(si, offset))
>> -			goto checks;
>>  		if (unlikely(--latency_ration < 0)) {
>>  			cond_resched();
>>  			latency_ration = LATENCY_LIMIT;
>>  			scanned_many = true;
>>  		}
>> +		if (swap_offset_available_and_locked(si, offset))
>> +			goto checks;
>>  	}
>>  	offset = si->lowest_bit;
>>  	while (offset < scan_base) {
>> -		if (swap_offset_available_and_locked(si, offset))
>> -			goto checks;
>>  		if (unlikely(--latency_ration < 0)) {
>>  			cond_resched();
>>  			latency_ration = LATENCY_LIMIT;
>>  			scanned_many = true;
>>  		}
>> +		if (swap_offset_available_and_locked(si, offset))
>> +			goto checks;
>>  		offset++;
>>  	}
>>  	spin_lock(&si->lock);
>
> But this does somewhat alter the `scanned_many' logic.  We'll now set
> 'scanned_many` earlier.  What are the effects of this?
>
> The ed43af10975eef7e changelog outlines tests which could be performed
> to ensure we aren't regressing from this.

Per my understanding, this will not influence `scanned_many` logic
much.  Because `scanned_many` flag will be set just a little earlier
(one less slot).

Best Regards,
Huang, Ying