[PATCH 04/18] mm: numa: Do not migrate or account for hinting faults on the zero page

[Mel Gorman]

The zero page is not replicated between nodes and is often shared
between processes. The data is read-only and likely to be cached in
local CPUs if heavily accessed meaning that the remote memory access
cost is less of a concern. This patch stops accounting for numa hinting
faults on the zero page in both terms of counting faults and scheduling
tasks on nodes.

Signed-off-by: Mel Gorman <mgorman@xxxxxxx>
---
 mm/huge_memory.c | 9 +++++++++
 mm/memory.c      | 7 ++++++-
 2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index e4a79fa..ec938ed 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1302,6 +1302,15 @@ int do_huge_pmd_numa_page(struct mm_struct *mm, struct vm_area_struct *vma,
 
 	page = pmd_page(pmd);
 	get_page(page);
+
+	/*
+	 * Do not account for faults against the huge zero page. The read-only
+	 * data is likely to be read-cached on the local CPUs and it is less
+	 * useful to know about local versus remote hits on the zero page.
+	 */
+	if (is_huge_zero_pfn(page_to_pfn(page)))
+		goto clear_pmdnuma;
+
 	src_nid = numa_node_id();
 	count_vm_numa_event(NUMA_HINT_FAULTS);
 	if (src_nid == page_to_nid(page))
diff --git a/mm/memory.c b/mm/memory.c
index ba94dec..6c6f6b0 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3560,8 +3560,13 @@ int do_numa_page(struct mm_struct *mm, struct vm_area_struct *vma,
 	set_pte_at(mm, addr, ptep, pte);
 	update_mmu_cache(vma, addr, ptep);
 
+	/*
+	 * Do not account for faults against the zero page. The read-only data
+	 * is likely to be read-cached on the local CPUs and it is less useful
+	 * to know about local versus remote hits on the zero page.
+	 */
 	page = vm_normal_page(vma, addr, pte);
-	if (!page) {
+	if (!page || is_zero_pfn(page_to_pfn(page))) {
 		pte_unmap_unlock(ptep, ptl);
 		return 0;
 	}
-- 
1.8.1.4

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