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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