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SubjectRe: [PATCH v6] mm/migrate: split source folio if it is on deferred split list
From
On 26.03.24 16:00, Zi Yan wrote:
> From: Zi Yan <ziy@nvidia.com>
>
> 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 v5:
> 1. Fixed an error in migrate_misplaced_folio() reported by Baolin Wang[3].
>
> 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@nvidia.com/
> [2]: https://lore.kernel.org/linux-mm/Ze_P6xagdTbcu1Kz@casper.infradead.org/
> [3]: https://lore.kernel.org/linux-mm/df9a644c-a007-46ac-98e3-61d4014fcfff@linux.alibaba.com/
>
> Fixes: 616b8371539a ("mm: thp: enable thp migration in generic path")
> Signed-off-by: Zi Yan <ziy@nvidia.com>
> ---
> mm/migrate.c | 23 +++++++++++++++++++++++
> 1 file changed, 23 insertions(+)
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 1dbe5bd927de..a31aa75d223d 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, split_folios) == 0) {
> + 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

Acked-by: David Hildenbrand <david@redhat.com>

--
Cheers,

David / dhildenb


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