From a1a46184e34cfd0764f06a54870defa052b0a094 Mon Sep 17 00:00:00 2001 From: Mel Gorman Date: Mon, 7 Oct 2013 11:28:50 +0100 Subject: [PATCH] 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 Reviewed-by: Rik van Riel Cc: Andrea Arcangeli Cc: Johannes Weiner Cc: Srikar Dronamraju [ Correct use of is_huge_zero_page] Signed-off-by: Peter Zijlstra Link: http://lkml.kernel.org/r/1381141781-10992-13-git-send-email-mgorman@suse.de Signed-off-by: Ingo Molnar --- 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 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); -- 2.20.1