Re: [PATCH 1/2] mm: compaction: limit the suitable target page order to be less than cc->order

[Zi Yan]

On 12 Feb 2024, at 4:13, Vlastimil Babka wrote:

> On 1/22/24 14:01, Baolin Wang wrote:
>> It can not improve the fragmentation if we isolate the target free pages
>> exceeding cc->order, especially when the cc->order is less than pageblock_order.
>> For example, suppose the pageblock_order is MAX_ORDER (size is 4M) and cc->order
>> is 2M THP size, we should not isolate other 2M free pages to be the migration
>> target, which can not improve the fragmentation.
>>
>> Moreover this is also applicable for large folio compaction.
>
> So why not Cc: Zi Yan? (done)
>

Thanks.

Hi Baolin,

How often do you see this happening?

>> Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
>
> I doubt this will make much difference, because if such a larger order free
> page exists, we shouldn't have a reason to be compacting for a lower order
> in the first place?

Unless kswapd gets us such a free block in the background right after
get_page_from_freelist() and before compaction finishes in the allocation
slow path.

If this happens often and cc->order is not -1, it might be better to stop
compaction and get_page_from_freelist() to save cycles on unnecessary pfn
scanning. For completeness, when cc->order == -1, the logic does not change.


>
>> ---
>>  mm/compaction.c | 4 +++-
>>  1 file changed, 3 insertions(+), 1 deletion(-)
>>
>> diff --git a/mm/compaction.c b/mm/compaction.c
>> index 27ada42924d5..066b72b3471a 100644
>> --- a/mm/compaction.c
>> +++ b/mm/compaction.c
>> @@ -1346,12 +1346,14 @@ static bool suitable_migration_target(struct compact_control *cc,
>>  {
>>  	/* If the page is a large free page, then disallow migration */
>>  	if (PageBuddy(page)) {
>> +		int order = cc->order > 0 ? cc->order : pageblock_order;
>> +
>>  		/*
>>  		 * We are checking page_order without zone->lock taken. But
>>  		 * the only small danger is that we skip a potentially suitable
>>  		 * pageblock, so it's not worth to check order for valid range.
>>  		 */
>> -		if (buddy_order_unsafe(page) >= pageblock_order)
>> +		if (buddy_order_unsafe(page) >= order)
>>  			return false;
>>  	}
>>


--
Best Regards,
Yan, Zi

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