Re: [PATCH 1/2] mm: compaction: consider the number of scanning compound pages in isolate fail path

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 3/15/2023 11:54 PM, Vlastimil Babka wrote:
> On 3/13/23 11:37, Baolin Wang wrote:
>> The commit b717d6b93b54 ("mm: compaction: include compound page count
>> for scanning in pageblock isolation") had added compound page statistics
>> for scanning in pageblock isolation, to make sure the number of scanned
>> pages are always larger than the number of isolated pages when isolating
>> mirgratable or free pageblock.
>>
>> However, when failed to isolate the pages when scanning the mirgratable or
>> free pageblock, the isolation failure path did not consider the scanning
>> statistics of the compound pages, which can show the incorrect number of
>> scanned pages in tracepoints or the vmstats to make people confusing about
>> the page scanning pressure in memory compaction.
>>
>> Thus we should take into account the number of scanning pages when failed
>> to isolate the compound pages to make the statistics accurate.
>>
>> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
>> ---
>>   mm/compaction.c | 6 +++---
>>   1 file changed, 3 insertions(+), 3 deletions(-)
>>
>> diff --git a/mm/compaction.c b/mm/compaction.c
>> index 5a9501e0ae01..c9d9ad958e2a 100644
>> --- a/mm/compaction.c
>> +++ b/mm/compaction.c
>> @@ -587,6 +587,7 @@ static unsigned long isolate_freepages_block(struct compact_control *cc,
>>   				blockpfn += (1UL << order) - 1;
>>   				cursor += (1UL << order) - 1;
>>   			}
>> +			nr_scanned += (1UL << order) - 1;
> 
> I'd rather put it in the block above that tests order < MAX_ORDER. Otherwise
> as the comments say, the value can be bogus as it's racy.

Right, thanks for pointing it out. Will do in next version.

> 
>>   			goto isolate_fail;
>>   		}
>>   
>> @@ -873,9 +874,8 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
>>   			cond_resched();
>>   		}
>>   
>> -		nr_scanned++;
>> -
>>   		page = pfn_to_page(low_pfn);
>> +		nr_scanned += compound_nr(page);
> 
> For the same reason, I'd rather leave the nr_scanned adjustment by order in
> the specific code blocks where we know/think we have a compound or huge page
> and have sanity checked the order/nr_pages, and not add an unchecked
> compound_nr() here.

OK. Sound reasonable to me. Thanks for your input.

>>   		/*
>>   		 * Check if the pageblock has already been marked skipped.
>> @@ -1077,6 +1077,7 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
>>   			 */
>>   			if (unlikely(PageCompound(page) && !cc->alloc_contig)) {
>>   				low_pfn += compound_nr(page) - 1;
>> +				nr_scanned += compound_nr(page) - 1;
>>   				SetPageLRU(page);
>>   				goto isolate_fail_put;
>>   			}
>> @@ -1097,7 +1098,6 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
>>   isolate_success_no_list:
>>   		cc->nr_migratepages += compound_nr(page);
>>   		nr_isolated += compound_nr(page);
>> -		nr_scanned += compound_nr(page) - 1;
>>   
>>   		/*
>>   		 * Avoid isolating too much unless this block is being