Re: [PATCH v5] mm/migrate: split source folio if it is on deferred split list

[Zi Yan]

On 26 Mar 2024, at 2:19, Baolin Wang wrote:

> On 2024/3/23 03:33, Zi Yan wrote:
>> From: Zi Yan <ziy@xxxxxxxxxx>
>>
>> If the source folio is on deferred split list, it is likely some subpages
>> are not used. Split it before migration to avoid migrating unused subpages.
>>
>> Commit 616b8371539a6 ("mm: thp: enable thp migration in generic path")
>> did not check if a THP is on deferred split list before migration, thus,
>> the destination THP is never put on deferred split list even if the source
>> THP might be. The opportunity of reclaiming free pages in a partially
>> mapped THP during deferred list scanning is lost, but no other harmful
>> consequence is present[1].
>>
>>  From v4:
>> 1. Simplify _deferred_list check without locking and do not count as
>>     migration failures. (per Matthew Wilcox)
>>
>>  From v3:
>> 1. Guarded deferred list code behind CONFIG_TRANSPARENT_HUGEPAGE to avoid
>>     compilation error (per SeongJae Park).
>>
>>  From v2:
>> 1. Split the source folio instead of migrating it (per Matthew Wilcox)[2].
>>
>>  From v1:
>> 1. Used dst to get correct deferred split list after migration
>>     (per Ryan Roberts).
>>
>> [1]: https://lore.kernel.org/linux-mm/03CE3A00-917C-48CC-8E1C-6A98713C817C@xxxxxxxxxx/
>> [2]: https://lore.kernel.org/linux-mm/Ze_P6xagdTbcu1Kz@xxxxxxxxxxxxxxxxxxxx/
>>
>> Fixes: 616b8371539a ("mm: thp: enable thp migration in generic path")
>> Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
>> ---
>>   mm/migrate.c | 23 +++++++++++++++++++++++
>>   1 file changed, 23 insertions(+)
>>
>> diff --git a/mm/migrate.c b/mm/migrate.c
>> index ab9856f5931b..6bd9319624a3 100644
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -1652,6 +1652,29 @@ static int migrate_pages_batch(struct list_head *from,
>>    			cond_resched();
>>  +			/*
>> +			 * The rare folio on the deferred split list should
>> +			 * be split now. It should not count as a failure.
>> +			 * Only check it without removing it from the list.
>> +			 * Since the folio can be on deferred_split_scan()
>> +			 * local list and removing it can cause the local list
>> +			 * corruption. Folio split process below can handle it
>> +			 * with the help of folio_ref_freeze().
>> +			 *
>> +			 * nr_pages > 2 is needed to avoid checking order-1
>> +			 * page cache folios. They exist, in contrast to
>> +			 * non-existent order-1 anonymous folios, and do not
>> +			 * use _deferred_list.
>> +			 */
>> +			if (nr_pages > 2 &&
>> +			   !list_empty(&folio->_deferred_list)) {
>> +				if (try_split_folio(folio, from) == 0) {
>
> IMO, we should move the split folios into the 'split_folios' list instead of the 'from' list, otherwise there might be unhandled folios remaining in the from list.

Can you elaborate on the actual situation you are thinking about? Thanks.

>
>> +					stats->nr_thp_split += is_thp;
>> +					stats->nr_split++;
>> +					continue;
>> +				}
>> +			}
>> +
>>   			/*
>>   			 * Large folio migration might be unsupported or
>>   			 * the allocation might be failed so we should retry
>>
>> base-commit: 08a487ab26d541a3bd0adaee144f684b724d233b


--
Best Regards,
Yan, Zi

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