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SubjectRe: [PATCH 1/4] vmstat: remove zone->lock from walk_zones_in_node
> * KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> [2009-12-28 16:47:22]:
>
> > The zone->lock is one of performance critical locks. Then, it shouldn't
> > be hold for long time. Currently, we have four walk_zones_in_node()
> > usage and almost use-case don't need to hold zone->lock.
> >
> > Thus, this patch move locking responsibility from walk_zones_in_node
> > to its sub function. Also this patch kill unnecessary zone->lock taking.
> >
> > Cc: Mel Gorman <mel@csn.ul.ie>
> > Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
> > ---
> > mm/vmstat.c | 8 +++++---
> > 1 files changed, 5 insertions(+), 3 deletions(-)
> >
> > diff --git a/mm/vmstat.c b/mm/vmstat.c
> > index 6051fba..a5d45bc 100644
> > --- a/mm/vmstat.c
> > +++ b/mm/vmstat.c
> > @@ -418,15 +418,12 @@ static void walk_zones_in_node(struct seq_file *m, pg_data_t *pgdat,
> > {
> > struct zone *zone;
> > struct zone *node_zones = pgdat->node_zones;
> > - unsigned long flags;
> >
> > for (zone = node_zones; zone - node_zones < MAX_NR_ZONES; ++zone) {
> > if (!populated_zone(zone))
> > continue;
> >
> > - spin_lock_irqsave(&zone->lock, flags);
> > print(m, pgdat, zone);
> > - spin_unlock_irqrestore(&zone->lock, flags);
> > }
> > }
> >
>
> > @@ -455,6 +452,7 @@ static void pagetypeinfo_showfree_print(struct seq_file *m,
> > pg_data_t *pgdat, struct zone *zone)
> > {
> > int order, mtype;
> > + unsigned long flags;
> >
> > for (mtype = 0; mtype < MIGRATE_TYPES; mtype++) {
> > seq_printf(m, "Node %4d, zone %8s, type %12s ",
> > @@ -468,8 +466,11 @@ static void pagetypeinfo_showfree_print(struct seq_file *m,
> >
> > area = &(zone->free_area[order]);
> >
> > + spin_lock_irqsave(&zone->lock, flags);
> > list_for_each(curr, &area->free_list[mtype])
> > freecount++;
> > + spin_unlock_irqrestore(&zone->lock, flags);
> > +
> > seq_printf(m, "%6lu ", freecount);
> > }
> > seq_putc(m, '\n');
> > @@ -709,6 +710,7 @@ static void zoneinfo_show_print(struct seq_file *m, pg_data_t *pgdat,
> > struct zone *zone)
> > {
> > int i;
> > +
> > seq_printf(m, "Node %d, zone %8s", pgdat->node_id, zone->name);
> > seq_printf(m,
> > "\n pages free %lu"
>
> While this is a noble cause, is printing all this information correct
> without the lock, I am not worried about old
> information, but incorrect data. Should the read side be rcu'ed. Is
> just removing the lock and accessing data safe across architectures?

Hm.
Actually,current memory-hotplug implementation change various zone data without zone->lock.
then, my patch doesn't cause regression. I'm not sure rcu protection is very worth or not.
but I think it can separate this patch.






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