Re: [PATCH v6 2/4] mm/compaction: enable compacting >0 order folios.

[Zi Yan]

On 20 Feb 2024, at 4:03, David Hildenbrand wrote:

> On 16.02.24 18:04, Zi Yan wrote:
>> From: Zi Yan <ziy@xxxxxxxxxx>
>>
>> migrate_pages() supports >0 order folio migration and during compaction,
>> even if compaction_alloc() cannot provide >0 order free pages,
>> migrate_pages() can split the source page and try to migrate the base
>> pages from the split.  It can be a baseline and start point for adding
>> support for compacting >0 order folios.
>>
>> Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
>> Suggested-by: Huang Ying <ying.huang@xxxxxxxxx>
>> Reviewed-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
>> Reviewed-by: Vlastimil Babka <vbabka@xxxxxxx>
>> Tested-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
>> Tested-by: Yu Zhao <yuzhao@xxxxxxxxxx>
>> Cc: Adam Manzanares <a.manzanares@xxxxxxxxxxx>
>> Cc: David Hildenbrand <david@xxxxxxxxxx>
>> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
>> Cc: Kemeng Shi <shikemeng@xxxxxxxxxxxxxxx>
>> Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
>> Cc: Luis Chamberlain <mcgrof@xxxxxxxxxx>
>> Cc: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
>> Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
>> Cc: Ryan Roberts <ryan.roberts@xxxxxxx>
>> Cc: Vishal Moola (Oracle) <vishal.moola@xxxxxxxxx>
>> Cc: Vlastimil Babka <vbabka@xxxxxxx>
>> Cc: Yin Fengwei <fengwei.yin@xxxxxxxxx>
>> ---
>>   mm/compaction.c | 66 ++++++++++++++++++++++++++++++++++++++-----------
>>   1 file changed, 52 insertions(+), 14 deletions(-)
>>
>> diff --git a/mm/compaction.c b/mm/compaction.c
>> index cc801ce099b4..aa6aad805c4d 100644
>> --- a/mm/compaction.c
>> +++ b/mm/compaction.c
>> @@ -816,6 +816,21 @@ static bool too_many_isolated(struct compact_control *cc)
>>   	return too_many;
>>   }
>>  +/*
>
>
> Can't you add these comments to the respective checks? Like
>
> static bool skip_isolation_on_order(int order, int target_order)
> {
> 	/*
> 	 * Unless we are performing global compaction (targert_order <
> 	 * 0), skip any folios that are larger than the target order: we
> 	 * wouldn't be here if we'd have a free folio with the desired
> 	 * target_order, so migrating this folio would likely fail
> 	 * later.
> 	 */
> 	if (target_order != -1 && order >= target_order)
> 		return true;
> 	/*
> 	 * We limit memory compaction to pageblocks and won't try
> 	 * creating free blocks of memory that are larger than that.
> 	 */
> 	return order >= pageblock_order;
> }
>
> Then, add a simple expressive function documentation (if really required) that doesn't contain all these details.
>

OK. No problem.

>> + * 1. if the page order is larger than or equal to target_order (i.e.,
>> + * cc->order and when it is not -1 for global compaction), skip it since
>> + * target_order already indicates no free page with larger than target_order
>> + * exists and later migrating it will most likely fail;
>> + *
>> + * 2. compacting > pageblock_order pages does not improve memory fragmentation,
>
> I'm pretty sure you meant "reduce" ?

Yes.

>
>> + * skip them;
>> + */
>> +static bool skip_isolation_on_order(int order, int target_order)
>> +{
>> +	return (target_order != -1 && order >= target_order) ||
>> +		order >= pageblock_order;
>> +}
>> +
>>   /**
>>    * isolate_migratepages_block() - isolate all migrate-able pages within
>>    *				  a single pageblock
>> @@ -947,7 +962,22 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
>>   			valid_page = page;
>>   		}
>>  -		if (PageHuge(page) && cc->alloc_contig) {
>> +		if (PageHuge(page)) {
>> +			/*
>> +			 * skip hugetlbfs if we are not compacting for pages
>> +			 * bigger than its order. THPs and other compound pages
>> +			 * are handled below.
>> +			 */
>> +			if (!cc->alloc_contig) {
>> +				const unsigned int order = compound_order(page);
>> +
>> +				if (order <= MAX_PAGE_ORDER) {
>> +					low_pfn += (1UL << order) - 1;
>> +					nr_scanned += (1UL << order) - 1;
>> +				}
>> +				goto isolate_fail;
>> +			}
>> +			/* for alloc_contig case */
>>   			if (locked) {
>>   				unlock_page_lruvec_irqrestore(locked, flags);
>>   				locked = NULL;
>> @@ -1008,21 +1038,24 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
>>   		}
>>    		/*
>> -		 * Regardless of being on LRU, compound pages such as THP and
>> -		 * hugetlbfs are not to be compacted unless we are attempting
>> -		 * an allocation much larger than the huge page size (eg CMA).
>> -		 * We can potentially save a lot of iterations if we skip them
>> -		 * at once. The check is racy, but we can consider only valid
>> -		 * values and the only danger is skipping too much.
>> +		 * Regardless of being on LRU, compound pages such as THP
>> +		 * (hugetlbfs is handled above) are not to be compacted unless
>> +		 * we are attempting an allocation larger than the compound
>> +		 * page size. We can potentially save a lot of iterations if we
>> +		 * skip them at once. The check is racy, but we can consider
>> +		 * only valid values and the only danger is skipping too much.
>>   		 */
>>   		if (PageCompound(page) && !cc->alloc_contig) {
>>   			const unsigned int order = compound_order(page);
>>  -			if (likely(order <= MAX_PAGE_ORDER)) {
>> -				low_pfn += (1UL << order) - 1;
>> -				nr_scanned += (1UL << order) - 1;
>> +			/* Skip based on page order and compaction target order. */
>> +			if (skip_isolation_on_order(order, cc->order)) {
>> +				if (order <= MAX_PAGE_ORDER) {
>> +					low_pfn += (1UL << order) - 1;
>> +					nr_scanned += (1UL << order) - 1;
>> +				}
>> +				goto isolate_fail;
>>   			}
>> -			goto isolate_fail;
>>   		}
>>    		/*
>> @@ -1165,10 +1198,11 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
>>   			}
>>    			/*
>> -			 * folio become large since the non-locked check,
>> -			 * and it's on LRU.
>> +			 * Check LRU folio order under the lock
>>   			 */
>> -			if (unlikely(folio_test_large(folio) && !cc->alloc_contig)) {
>> +			if (unlikely(skip_isolation_on_order(folio_order(folio),
>> +							     cc->order) &&
>> +				     !cc->alloc_contig)) {
>>   				low_pfn += folio_nr_pages(folio) - 1;
>>   				nr_scanned += folio_nr_pages(folio) - 1;
>>   				folio_set_lru(folio);
>> @@ -1788,6 +1822,10 @@ static struct folio *compaction_alloc(struct folio *src, unsigned long data)
>>   	struct compact_control *cc = (struct compact_control *)data;
>>   	struct folio *dst;
>>  +	/* this makes migrate_pages() split the source page and retry */
>> +	if (folio_test_large(src) > 0)
>> +		return NULL;
>
> Why the "> 0 " check ? Either it's large or it isn't.
Will fix it.

> Apart from that LGTM, but I am no compaction expert.

Thanks.


--
Best Regards,
Yan, Zi

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