[PATCH 12/63] 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 prevents trapping faults on the zero pages. For
tasks using the zero page this will reduce the number of PTE updates,
TLB flushes and hinting faults.

[peterz@xxxxxxxxxxxxx: Correct use of is_huge_zero_page]
Signed-off-by: Mel Gorman <mgorman@xxxxxxx>
---
 mm/huge_memory.c | 10 +++++++++-
 mm/memory.c      |  1 +
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index ac5af18..dd94f4e 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1303,6 +1303,7 @@ int do_huge_pmd_numa_page(struct mm_struct *mm, struct vm_area_struct *vma,
 		goto out_unlock;
 
 	page = pmd_page(pmd);
+	BUG_ON(is_huge_zero_page(page));
 	page_nid = page_to_nid(page);
 	count_vm_numa_event(NUMA_HINT_FAULTS);
 	if (page_nid == this_nid)
@@ -1493,8 +1494,15 @@ int change_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
 		} else {
 			struct page *page = pmd_page(*pmd);
 
-			/* only check non-shared pages */
+			/*
+			 * Only check non-shared pages. Do not trap faults
+			 * against the zero page. The read-only data is likely
+			 * to be read-cached on the local CPU cache and it is
+			 * less useful to know about local vs remote hits on
+			 * the zero page.
+			 */
 			if (page_mapcount(page) == 1 &&
+			    !is_huge_zero_page(page) &&
 			    !pmd_numa(*pmd)) {
 				entry = pmdp_get_and_clear(mm, addr, pmd);
 				entry = pmd_mknuma(entry);
diff --git a/mm/memory.c b/mm/memory.c
index c20f872..86c3caf 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3575,6 +3575,7 @@ int do_numa_page(struct mm_struct *mm, struct vm_area_struct *vma,
 		pte_unmap_unlock(ptep, ptl);
 		return 0;
 	}
+	BUG_ON(is_zero_pfn(page_to_pfn(page)));
 
 	page_nid = page_to_nid(page);
 	target_nid = numa_migrate_prep(page, vma, addr, page_nid);
-- 
1.8.1.4

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