[PATCH] mm, compaction: Skip all pinned pages during scan

[Khalid Aziz]

Pinned pages can not be migrated. Currently as
isolate_migratepages_block() scans pages for compaction, it skips
any pinned anonymous pages. All pinned pages should be skipped and
not just the anonymous pinned pages. This patch adds a check for
pinned page by comparing its refcount with mapcount and accounts for
possible extra refcounts. This was seen as a real issue on a
customer workload where a large number of pages were pinned by vfio
on the host and any attempts to allocate hugepages resulted in
significant amount of cpu time spent in either direct compaction or
in kcompatd scanning vfio pinned pages over and over again that can
not be migrated.

Signed-off-by: Khalid Aziz <khalid.aziz@xxxxxxxxxx>
Suggested-by: Steve Sistare <steven.sistare@xxxxxxxxxx>
---
 mm/compaction.c | 33 +++++++++++++++++++++++++++++----
 1 file changed, 29 insertions(+), 4 deletions(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index 5a9501e0ae01..d1371fd75391 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -764,6 +764,32 @@ static bool too_many_isolated(pg_data_t *pgdat)
 	return too_many;
 }
 
+/*
+ * Check if this base page should be skipped from isolation because
+ * it is pinned. This function is called for regular pages only, and not
+ * for THP or hugetlbfs pages. This code is inspired by similar code
+ * in migrate_vma_check_page(), can_split_folio() and
+ * folio_migrate_mapping()
+ */
+static inline bool is_pinned_page(struct page *page)
+{
+	unsigned long extra_refs;
+
+	/* anonymous page can have extra ref from page cache */
+	if (page_mapping(page))
+		extra_refs = 1 + page_has_private(page);
+	else
+		extra_refs = PageSwapCache(page) ? 1 : 0;
+
+	/*
+	 * This is an admittedly racy check but good enough to determine
+	 * if a page should be isolated
+	 */
+	if ((page_count(page) - extra_refs) > page_mapcount(page))
+		return true;
+	return false;
+}
+
 /**
  * isolate_migratepages_block() - isolate all migrate-able pages within
  *				  a single pageblock
@@ -992,12 +1018,11 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
 			goto isolate_fail;
 
 		/*
-		 * Migration will fail if an anonymous page is pinned in memory,
-		 * so avoid taking lru_lock and isolating it unnecessarily in an
-		 * admittedly racy check.
+		 * Migration will fail if a page is pinned in memory,
+		 * so avoid taking lru_lock and isolating it unnecessarily
 		 */
 		mapping = page_mapping(page);
-		if (!mapping && (page_count(page) - 1) > total_mapcount(page))
+		if (is_pinned_page(page))
 			goto isolate_fail_put;
 
 		/*
-- 
2.37.2