[PATCH 13/18] mm: numa: Scan pages with elevated page_mapcount

[Mel Gorman <mgorman@suse.de>]

Currently automatic NUMA balancing is unable to distinguish between false
shared versus private pages except by ignoring pages with an elevated
page_mapcount entirely. This avoids shared pages bouncing between the
nodes whose task is using them but that is ignored quite a lot of data.

This patch kicks away the training wheels in preparation for adding support
for identifying shared/private pages is now in place. The ordering is so
that the impact of the shared/private detection can be easily measured. Note
that the patch does not migrate shared, file-backed within vmas marked
VM_EXEC as these are generally shared library pages. Migrating such pages
is not beneficial as there is an expectation they are read-shared between
caches and iTLB and iCache pressure is generally low.

Signed-off-by: Mel Gorman <mgorman@suse.de>
---
 include/linux/migrate.h |  7 ++++---
 mm/memory.c             |  7 ++-----
 mm/migrate.c            | 17 ++++++-----------
 mm/mprotect.c           |  4 +---
 4 files changed, 13 insertions(+), 22 deletions(-)

diff --git a/include/linux/migrate.h b/include/linux/migrate.h
index a405d3dc..e7e26af 100644
--- a/include/linux/migrate.h
+++ b/include/linux/migrate.h
@@ -92,11 +92,12 @@ static inline int migrate_huge_page_move_mapping(struct address_space *mapping,
 #endif /* CONFIG_MIGRATION */
 
 #ifdef CONFIG_NUMA_BALANCING
-extern int migrate_misplaced_page(struct page *page, int node);
-extern int migrate_misplaced_page(struct page *page, int node);
+extern int migrate_misplaced_page(struct page *page,
+				  struct vm_area_struct *vma, int node);
 extern bool migrate_ratelimited(int node);
 #else
-static inline int migrate_misplaced_page(struct page *page, int node)
+static inline int migrate_misplaced_page(struct page *page,
+					 struct vm_area_struct *vma, int node)
 {
 	return -EAGAIN; /* can't migrate now */
 }
diff --git a/mm/memory.c b/mm/memory.c
index ab933be..62ae8a7 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3586,7 +3586,7 @@ int do_numa_page(struct mm_struct *mm, struct vm_area_struct *vma,
 	}
 
 	/* Migrate to the requested node */
-	migrated = migrate_misplaced_page(page, target_nid);
+	migrated = migrate_misplaced_page(page, vma, target_nid);
 	if (migrated)
 		current_nid = target_nid;
 
@@ -3651,9 +3651,6 @@ static int do_pmd_numa_page(struct mm_struct *mm, struct vm_area_struct *vma,
 		page = vm_normal_page(vma, addr, pteval);
 		if (unlikely(!page))
 			continue;
-		/* only check non-shared pages */
-		if (unlikely(page_mapcount(page) != 1))
-			continue;
 
 		/*
 		 * Note that the NUMA fault is later accounted to either
@@ -3671,7 +3668,7 @@ static int do_pmd_numa_page(struct mm_struct *mm, struct vm_area_struct *vma,
 
 		/* Migrate to the requested node */
 		pte_unmap_unlock(pte, ptl);
-		migrated = migrate_misplaced_page(page, target_nid);
+		migrated = migrate_misplaced_page(page, vma, target_nid);
 		if (migrated)
 			curr_nid = target_nid;
 		task_numa_fault(last_nid, curr_nid, 1, migrated);
diff --git a/mm/migrate.c b/mm/migrate.c
index 3bbaf5d..23f8122 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1579,7 +1579,8 @@ int numamigrate_isolate_page(pg_data_t *pgdat, struct page *page)
  * node. Caller is expected to have an elevated reference count on
  * the page that will be dropped by this function before returning.
  */
-int migrate_misplaced_page(struct page *page, int node)
+int migrate_misplaced_page(struct page *page, struct vm_area_struct *vma,
+			   int node)
 {
 	pg_data_t *pgdat = NODE_DATA(node);
 	int isolated;
@@ -1587,10 +1588,11 @@ int migrate_misplaced_page(struct page *page, int node)
 	LIST_HEAD(migratepages);
 
 	/*
-	 * Don't migrate pages that are mapped in multiple processes.
-	 * TODO: Handle false sharing detection instead of this hammer
+	 * Don't migrate file pages that are mapped in multiple processes
+	 * with execute permissions as they are probably shared libraries.
 	 */
-	if (page_mapcount(page) != 1)
+	if (page_mapcount(page) != 1 && page_is_file_cache(page) &&
+	    (vma->vm_flags & VM_EXEC))
 		goto out;
 
 	/*
@@ -1641,13 +1643,6 @@ int migrate_misplaced_transhuge_page(struct mm_struct *mm,
 	int page_lru = page_is_file_cache(page);
 
 	/*
-	 * Don't migrate pages that are mapped in multiple processes.
-	 * TODO: Handle false sharing detection instead of this hammer
-	 */
-	if (page_mapcount(page) != 1)
-		goto out_dropref;
-
-	/*
 	 * Rate-limit the amount of data that is being migrated to a node.
 	 * Optimal placement is no good if the memory bus is saturated and
 	 * all the time is being spent migrating!
diff --git a/mm/mprotect.c b/mm/mprotect.c
index 94722a4..cacc64a 100644
--- a/mm/mprotect.c
+++ b/mm/mprotect.c
@@ -69,9 +69,7 @@ static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
 					if (last_nid != this_nid)
 						all_same_node = false;
 
-					/* only check non-shared pages */
-					if (!pte_numa(oldpte) &&
-					    page_mapcount(page) == 1) {
+					if (!pte_numa(oldpte)) {
 						ptent = pte_mknuma(ptent);
 						updated = true;
 					}
-- 
1.8.1.4

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