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Subject[PATCH 12/63] mm: numa: Do not migrate or account for hinting faults on the zero page
Date
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@infradead.org: Correct use of is_huge_zero_page]
Signed-off-by: Mel Gorman <mgorman@suse.de>
---
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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