On Wed, Sep 01, 2010 at 08:37:41AM +0900, KOSAKI Motohiro wrote:
> > +#ifdef CONFIG_SMP
> > +/* Called when a more accurate view of NR_FREE_PAGES is needed */
> > +unsigned long zone_nr_free_pages(struct zone *zone)
> > +{
> > + unsigned long nr_free_pages = zone_page_state(zone, NR_FREE_PAGES);
> > +
> > + /*
> > + * While kswapd is awake, it is considered the zone is under some
> > + * memory pressure. Under pressure, there is a risk that
> > + * per-cpu-counter-drift will allow the min watermark to be breached
> > + * potentially causing a live-lock. While kswapd is awake and
> > + * free pages are low, get a better estimate for free pages
> > + */
> > + if (nr_free_pages < zone->percpu_drift_mark &&
> > + !waitqueue_active(&zone->zone_pgdat->kswapd_wait)) {
> > + int cpu;
> > +
> > + for_each_online_cpu(cpu) {
> > + struct per_cpu_pageset *pset;
> > +
> > + pset = per_cpu_ptr(zone->pageset, cpu);
> > + nr_free_pages += pset->vm_stat_diff[NR_FREE_PAGES];
>
> If my understanding is correct, we have no lock when reading pset->vm_stat_diff.
> It mean nr_free_pages can reach negative value at very rarely race. boundary
> check is necessary?
>
True, well spotted.
How about the following? It records a delta and checks if delta is negative
and would cause underflow.
unsigned long zone_nr_free_pages(struct zone *zone)
{
unsigned long nr_free_pages = zone_page_state(zone, NR_FREE_PAGES);
long delta = 0;
/*
* While kswapd is awake, it is considered the zone is under some
* memory pressure. Under pressure, there is a risk that
* per-cpu-counter-drift will allow the min watermark to be breached
* potentially causing a live-lock. While kswapd is awake and
* free pages are low, get a better estimate for free pages
*/
if (nr_free_pages < zone->percpu_drift_mark &&
!waitqueue_active(&zone->zone_pgdat->kswapd_wait)) {
int cpu;
for_each_online_cpu(cpu) {
struct per_cpu_pageset *pset;
pset = per_cpu_ptr(zone->pageset, cpu);
delta += pset->vm_stat_diff[NR_FREE_PAGES];
}
}
/* Watch for underflow */
if (delta < 0 && abs(delta) > nr_free_pages)
delta = -nr_free_pages;
return nr_free_pages + delta;
}
--
Mel Gorman
Part-time Phd Student Linux Technology Center
University of Limerick IBM Dublin Software Lab
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