Re: [v4 PATCH 2/2] mm: vmscan: correct some vmscan counters for THP swapout

[Yang Shi <yang.shi@linux.alibaba.com>]

On 5/23/19 11:51 PM, Hillf Danton wrote:
> On Thu, 23 May 2019 10:27:38 +0800 Yang Shi wrote:
>> @ -1642,14 +1650,14 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
>>   	unsigned long nr_zone_taken[MAX_NR_ZONES] = { 0 };
>>   	unsigned long nr_skipped[MAX_NR_ZONES] = { 0, };
>>   	unsigned long skipped = 0;
>> -	unsigned long scan, total_scan, nr_pages;
>> +	unsigned long scan, total_scan;
>> +	unsigned long nr_pages;
> Change for no earn:)

Aha, yes.

>
>>   	LIST_HEAD(pages_skipped);
>>   	isolate_mode_t mode = (sc->may_unmap ? 0 : ISOLATE_UNMAPPED);
>>   
>> +	total_scan = 0;
>>   	scan = 0;
>> -	for (total_scan = 0;
>> -	     scan < nr_to_scan && nr_taken < nr_to_scan && !list_empty(src);
>> -	     total_scan++) {
>> +	while (scan < nr_to_scan && !list_empty(src)) {
>>   		struct page *page;
> AFAICS scan currently prevents us from looping for ever, while nr_taken bails
> us out once we get what's expected, so I doubt it makes much sense to cut
> nr_taken off.

It is because "scan < nr_to_scan && nr_taken >= nr_to_scan" is 
impossible now with the units fixed.

>>   
>>   		page = lru_to_page(src);
>> @@ -1657,9 +1665,12 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
>>   
>>   		VM_BUG_ON_PAGE(!PageLRU(page), page);
>>   
>> +		nr_pages = 1 << compound_order(page);
>> +		total_scan += nr_pages;
>> +
>>   		if (page_zonenum(page) > sc->reclaim_idx) {
>>   			list_move(&page->lru, &pages_skipped);
>> -			nr_skipped[page_zonenum(page)]++;
>> +			nr_skipped[page_zonenum(page)] += nr_pages;
>>   			continue;
>>   		}
>>   
>> @@ -1669,10 +1680,9 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
>>   		 * ineligible pages.  This causes the VM to not reclaim any
>>   		 * pages, triggering a premature OOM.
>>   		 */
>> -		scan++;
>> +		scan += nr_pages;
> The comment looks to defy the change if we fail to add a huge page to
> the dst list; otherwise nr_taken knows how to do the right thing. What
> I prefer is to let scan to do one thing a time.

I don't get your point. Do you mean the comment "Do not count skipped 
pages because that makes the function return with no isolated pages if 
the LRU mostly contains ineligible pages."? I'm supposed the comment is 
used to explain why not count skipped page.

>
>>   		switch (__isolate_lru_page(page, mode)) {
>>   		case 0:
>> -			nr_pages = hpage_nr_pages(page);
>>   			nr_taken += nr_pages;
>>   			nr_zone_taken[page_zonenum(page)] += nr_pages;
>>   			list_move(&page->lru, dst);
>> -- 
>> 1.8.3.1
>>
> Best Regards
> Hillf