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

Zi Yan
2023-11-13T17:02:33Z

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;
}

/* The folio is taken off the LRU */
@@ -1764,6 +1787,10 @@ static struct folio *compaction_alloc(struct folio *src, unsigned long data)
struct compact_control *cc = (struct compact_control *)data;
struct folio *dst;

+ /* this makes migrate_pages() split the source page and retry */
+ if (folio_order(src) > 0)
+ return NULL;
+
if (list_empty(&cc->freepages)) {
isolate_freepages(cc);

--
2.42.0