Re: [RFC PATCH v3 2/8] mm: compaction: handle non-lru compound pages properly in isolate_migratepages_block().

[Zi Yan]

On 12 Jan 2022, at 6:01, David Hildenbrand wrote:

> On 05.01.22 22:47, Zi Yan wrote:
>> From: Zi Yan <ziy@xxxxxxxxxx>
>>
>> In isolate_migratepages_block(), a !PageLRU tail page can be encountered
>> when the page is larger than a pageblock. Use compound head page for the
>> checks inside and skip the entire compound page when isolation succeeds.
>>
>
> This will currently never happen, due to the way we always isolate
> MAX_ORDER -1 ranges, correct?

You are right.

>
> Better note that in the patch description, because currently it reads
> like it's an actual fix "can be encountered".
>

Will do. This is a preparation patch for the upcoming commits.


>> Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
>> ---
>>  mm/compaction.c | 10 +++++++---
>>  1 file changed, 7 insertions(+), 3 deletions(-)
>>
>> diff --git a/mm/compaction.c b/mm/compaction.c
>> index b4e94cda3019..ad9053fbbe06 100644
>> --- a/mm/compaction.c
>> +++ b/mm/compaction.c
>> @@ -979,19 +979,23 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
>>  		 * Skip any other type of page
>>  		 */
>>  		if (!PageLRU(page)) {
>> +			struct page *head = compound_head(page);
>>  			/*
>>  			 * __PageMovable can return false positive so we need
>>  			 * to verify it under page_lock.
>>  			 */
>> -			if (unlikely(__PageMovable(page)) &&
>> -					!PageIsolated(page)) {
>> +			if (unlikely(__PageMovable(head)) &&
>> +					!PageIsolated(head)) {
>>  				if (locked) {
>>  					unlock_page_lruvec_irqrestore(locked, flags);
>>  					locked = NULL;
>>  				}
>>
>> -				if (!isolate_movable_page(page, isolate_mode))
>> +				if (!isolate_movable_page(head, isolate_mode)) {
>> +					low_pfn += (1 << compound_order(head)) - 1 - (page - head);
>> +					page = head;
>>  					goto isolate_success;
>> +				}
>>  			}
>>
>>  			goto isolate_fail;
>
>
> -- 
> Thanks,
>
> David / dhildenb


--
Best Regards,
Yan, Zi

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