[PATCH v1 3/4] mm/compaction: optimize >0 order folio compaction with free page split.

Zi Yan
2023-11-13T17:02:30Z

From: Zi Yan <ziy@nvidia.com>

During migration in a memory compaction, free pages are placed in an array
of page lists based on their order. But the desired free page order (i.e.,
the order of a source page) might not be always present, thus leading to
migration failures. Split a high order free pages when source migration
page has a lower order to increase migration successful rate.

Note: merging free pages when a migration fails and a lower order free
page is returned via compaction_free() is possible, but there is too much
work. Since the free pages are not buddy pages, it is hard to identify
these free pages using existing PFN-based page merging algorithm.

Signed-off-by: Zi Yan <ziy@nvidia.com>
---
mm/compaction.c | 40 +++++++++++++++++++++++++++++++++++++++-
1 file changed, 39 insertions(+), 1 deletion(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index ec6b5cc7e907..9c083e6b399a 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -1806,9 +1806,46 @@ static struct folio *compaction_alloc(struct folio *src, unsigned long data)
struct compact_control *cc = (struct compact_control *)data;
struct folio *dst;
int order = folio_order(src);
+ bool has_isolated_pages = false;

+again:
if (!cc->freepages[order].nr_pages) {
- isolate_freepages(cc);
+ int i;
+
+ for (i = order + 1; i <= MAX_ORDER; i++) {
+ if (cc->freepages[i].nr_pages) {
+ struct page *freepage =
+ list_first_entry(&cc->freepages[i].pages,
+ struct page, lru);
+
+ int start_order = i;
+ unsigned long size = 1 << start_order;
+
+ list_del(&freepage->lru);
+ cc->freepages[i].nr_pages--;
+
+ while (start_order > order) {
+ start_order--;
+ size >>= 1;
+
+ list_add(&freepage[size].lru,
+ &cc->freepages[start_order].pages);
+ cc->freepages[start_order].nr_pages++;
+ set_page_private(&freepage[size], start_order);
+ }
+ post_alloc_hook(freepage, order, __GFP_MOVABLE);
+ if (order)
+ prep_compound_page(freepage, order);
+ dst = page_folio(freepage);
+ goto done;
+ }
+ }
+ if (!has_isolated_pages) {
+ isolate_freepages(cc);
+ has_isolated_pages = true;
+ goto again;
+ }
+
if (!cc->freepages[order].nr_pages)
return NULL;
}
@@ -1819,6 +1856,7 @@ static struct folio *compaction_alloc(struct folio *src, unsigned long data)
post_alloc_hook(&dst->page, order, __GFP_MOVABLE);
if (order)
prep_compound_page(&dst->page, order);
+done:
cc->nr_freepages -= 1 << order;
return page_rmappable_folio(&dst->page);
}
--
2.42.0