On Wed, Apr 14, 2010 at 12:24:59AM +0900, Minchan Kim wrote:
> alloc_pages_node is called with cpu_to_node(cpu).
> I think cpu_to_node(cpu) never returns -1.
> (But I am not sure we need double check.)
>
> So we can use alloc_pages_exact_node instead of alloc_pages_node.
> It could avoid comparison and branch as 6484eb3e2a81807722 tried.
>
Well, numa_node_id() is implemented as
#ifndef numa_node_id
#define numa_node_id() (cpu_to_node(raw_smp_processor_id()))
#endif
and the mapping table on x86 at least is based on possible CPUs in
init_cpu_to_node() leaves the mapping as 0 if the APIC is bad or the numa
node is reported in apicid_to_node as -1. It would appear on power that
the node will be 0 for possible CPUs as well.
Hence, I believe this to be safe but a confirmation from Tejun would be
nice. I would continue digging but this looks like an initialisation path
so I'll move on to the next patch rather than spending more time.
> Cc: Tejun Heo <tj@xxxxxxxxxx>
> Cc: Christoph Lameter <cl@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Minchan Kim <minchan.kim@xxxxxxxxx>
> ---
> mm/percpu.c | 2 +-
> 1 files changed, 1 insertions(+), 1 deletions(-)
>
> diff --git a/mm/percpu.c b/mm/percpu.c
> index 768419d..ec3e671 100644
> --- a/mm/percpu.c
> +++ b/mm/percpu.c
> @@ -720,7 +720,7 @@ static int pcpu_alloc_pages(struct pcpu_chunk *chunk,
> for (i = page_start; i < page_end; i++) {
> struct page **pagep = &pages[pcpu_page_idx(cpu, i)];
>
> - *pagep = alloc_pages_node(cpu_to_node(cpu), gfp, 0);
> + *pagep = alloc_pages_exact_node(cpu_to_node(cpu), gfp, 0);
> if (!*pagep) {
> pcpu_free_pages(chunk, pages, populated,
> page_start, page_end);
> --
> 1.7.0.5
>
--
Mel Gorman
Part-time Phd Student Linux Technology Center
University of Limerick IBM Dublin Software Lab
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