mm: numa: Do not migrate or account for hinting faults on the zero page

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.

Signed-off-by: Mel Gorman <mgorman@suse.de>
Reviewed-by: Rik van Riel <riel@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
[ Correct use of is_huge_zero_page]
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1381141781-10992-13-git-send-email-mgorman@suse.de
Signed-off-by: Ingo Molnar <mingo@kernel.org>

2 files changed, 10 insertions, 1 deletions

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index de8d5cfc2bf2..8677dbf31c2e 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1291,6 +1291,7 @@ 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)
@@ -1481,8 +1482,15 @@ 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 42ae82ee04c1..ed51f15136ee 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3564,6 +3564,7 @@ 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);