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

[Peter Zijlstra]

On Mon, Jul 15, 2013 at 04:20:06PM +0100, Mel Gorman wrote:
> 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))

And because of:

  5918d10 thp: fix huge zero page logic for page with pfn == 0

--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1308,7 +1308,7 @@ int do_huge_pmd_numa_page(struct mm_stru
 	 * 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)))
+	if (is_huge_zero_page(page))
 		goto clear_pmdnuma;
 
 	src_nid = numa_node_id();

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