Re: [PATCH v1 1/4] mm/compaction: enable compacting >0 order folios.

Baolin Wang
2023-11-20T09:18:26Z

On 11/14/2023 1:01 AM, Zi Yan wrote:
> From: Zi Yan <ziy@nvidia.com>
>
> migrate_pages() supports >0 order folio migration and during compaction,
> even if compaction_alloc() cannot provide >0 order free pages,
> migrate_pages() can split the source page and try to migrate the base pages
> from the split. It can be a baseline and start point for adding support for
> compacting >0 order folios.
>
> Suggested-by: Huang Ying <ying.huang@intel.com>
> Signed-off-by: Zi Yan <ziy@nvidia.com>
> ---
> mm/compaction.c | 57 ++++++++++++++++++++++++++++++++++++-------------
> 1 file changed, 42 insertions(+), 15 deletions(-)
>
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 01ba298739dd..5217dd35b493 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -816,6 +816,21 @@ static bool too_many_isolated(struct compact_control *cc)
> return too_many;
> }
>
> +/*
> + * 1. if the page order is larger than or equal to target_order (i.e.,
> + * cc->order and when it is not -1 for global compaction), skip it since
> + * target_order already indicates no free page with larger than target_order
> + * exists and later migrating it will most likely fail;
> + *
> + * 2. compacting > pageblock_order pages does not improve memory fragmentation,
> + * skip them;
> + */
> +static bool skip_isolation_on_order(int order, int target_order)
> +{
> + return (target_order != -1 && order >= target_order) ||
> + order >= pageblock_order;
> +}
> +
> /**
> * isolate_migratepages_block() - isolate all migrate-able pages within
> * a single pageblock
> @@ -1009,7 +1024,7 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
> /*
> * Regardless of being on LRU, compound pages such as THP and
> * hugetlbfs are not to be compacted unless we are attempting
> - * an allocation much larger than the huge page size (eg CMA).
> + * an allocation larger than the compound page size.
> * We can potentially save a lot of iterations if we skip them
> * at once. The check is racy, but we can consider only valid
> * values and the only danger is skipping too much.
> @@ -1017,11 +1032,18 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
> if (PageCompound(page) && !cc->alloc_contig) {
> const unsigned int order = compound_order(page);
>
> - if (likely(order <= MAX_ORDER)) {
> - low_pfn += (1UL << order) - 1;
> - nr_scanned += (1UL << order) - 1;
> + /*
> + * Skip based on page order and compaction target order
> + * and skip hugetlbfs pages.
> + */
> + if (skip_isolation_on_order(order, cc->order) ||
> + PageHuge(page)) {
> + if (order <= MAX_ORDER) {
> + low_pfn += (1UL << order) - 1;
> + nr_scanned += (1UL << order) - 1;
> + }
> + goto isolate_fail;
> }
> - goto isolate_fail;
> }
>
> /*
> @@ -1144,17 +1166,18 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
> goto isolate_abort;
> }
> }
> + }
>
> - /*
> - * folio become large since the non-locked check,
> - * and it's on LRU.
> - */
> - if (unlikely(folio_test_large(folio) && !cc->alloc_contig)) {
> - low_pfn += folio_nr_pages(folio) - 1;
> - nr_scanned += folio_nr_pages(folio) - 1;
> - folio_set_lru(folio);
> - goto isolate_fail_put;
> - }
> + /*
> + * Check LRU folio order under the lock
> + */
> + if (unlikely(skip_isolation_on_order(folio_order(folio),
> + cc->order) &&
> + !cc->alloc_contig)) {
> + low_pfn += folio_nr_pages(folio) - 1;
> + nr_scanned += folio_nr_pages(folio) - 1;
> + folio_set_lru(folio);
> + goto isolate_fail_put;
> }

Why was this part moved out of the 'if (lruvec != locked)' block? If we
hold the lru lock, then we do not need to check again, right?