Re: [PATCH] mm/swap.c: workaround for_each_cpu() bug on UP kernel.

[Michal Hocko <mhocko@kernel.org>]

On Tue 12-02-19 19:25:46, Tetsuo Handa wrote:
> On 2019/02/12 19:11, Michal Hocko wrote:
> > This patch is ugly as hell! I do agree that for_each_cpu not working on
> > CONFIG_SMP=n sucks but why do we even care about lru_add_drain_all when
> > there is a single cpu? Why don't we simply do
> > 
> > diff --git a/mm/swap.c b/mm/swap.c
> > index aa483719922e..952f24b09070 100644
> > --- a/mm/swap.c
> > +++ b/mm/swap.c
> > @@ -660,6 +660,7 @@ static void lru_add_drain_per_cpu(struct work_struct *dummy)
> >  
> >  static DEFINE_PER_CPU(struct work_struct, lru_add_drain_work);
> >  
> > +#ifdef CONFIG_SMP
> >  /*
> >   * Doesn't need any cpu hotplug locking because we do rely on per-cpu
> >   * kworkers being shut down before our page_alloc_cpu_dead callback is
> > @@ -702,6 +703,10 @@ void lru_add_drain_all(void)
> >  
> >  	mutex_unlock(&lock);
> >  }
> > +#else
> > +#define lru_add_drain_all() lru_add_drain()
> > +
> > +#endif
> 
> If there is no need to evaluate the "if" conditions, I'm fine with this shortcut.

lru_add_drain does drain only pagevecs which have pages and so we do not
really have to duplicate the check. There is also no need to defer the
execution to the workqueue for a local cpu. So we are left with only the
lock to prevent parallel execution but the preemption disabling acts the
same purpose on UP so the approach should be equivalent from the
correctness point of view.
-- 
Michal Hocko
SUSE Labs