Re: [RFC PATCH 2/4] mm/compaction: optimize >0 order folio compaction with free page split.

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On 9/19/2023 1:20 AM, Zi Yan wrote:
On 18 Sep 2023, at 3:34, Baolin Wang wrote:

On 9/13/2023 12:28 AM, Zi Yan wrote:
From: Zi Yan <ziy@xxxxxxxxxx>

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@xxxxxxxxxx>
---
   mm/compaction.c | 40 +++++++++++++++++++++++++++++++++++++++-
   1 file changed, 39 insertions(+), 1 deletion(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index 868e92e55d27..45747ab5f380 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -1801,9 +1801,46 @@ 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_free) {
-		isolate_freepages(cc);
+		int i;
+
+		for (i = order + 1; i <= MAX_ORDER; i++) {
+			if (cc->freepages[i].nr_free) {
+				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_free--;
+
+				while (start_order > order) {
+					start_order--;
+					size >>= 1;
+
+					list_add(&freepage[size].lru,
+						&cc->freepages[start_order].pages);
+					cc->freepages[start_order].nr_free++;
+					set_page_private(&freepage[size], start_order);

IIUC, these split pages should also call functions to initialize? e.g. prep_compound_page()?

Not at this place. It is done right below and above "done" label. When free pages
are on cc->freepages, we want to keep them without being post_alloc_hook() or
prep_compound_page() processed for a possible future split. A free page is
only initialized when it is returned by compaction_alloc().

Ah, I see. Thanks for explanation.




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