Re: [PATCH -mm 5/5] swapcgroup (v3): implement force_empty

[yamamoto-jCdQPDEk3idL9jVzuh4AOg@public.gmane.org (YAMAMOTO Takashi)]

> Hi, Yamamoto-san.
> 
> Thank you for your comment.
> 
> On Fri,  4 Jul 2008 15:54:31 +0900 (JST), yamamoto-jCdQPDEk3idL9jVzuh4AOg@public.gmane.org (YAMAMOTO Takashi) wrote:
> > hi,
> > 
> > > +/*
> > > + * uncharge all the entries that are charged to the group.
> > > + */
> > > +void __swap_cgroup_force_empty(struct mem_cgroup *mem)
> > > +{
> > > +	struct swap_info_struct *p;
> > > +	int type;
> > > +
> > > +	spin_lock(&swap_lock);
> > > +	for (type = swap_list.head; type >= 0; type = swap_info[type].next) {
> > > +		p = swap_info + type;
> > > +
> > > +		if ((p->flags & SWP_ACTIVE) == SWP_ACTIVE) {
> > > +			unsigned int i = 0;
> > > +
> > > +			spin_unlock(&swap_lock);
> > 
> > what prevents the device from being swapoff'ed while you drop swap_lock?
> > 
> Nothing.
> 
> After searching the entry to be uncharged(find_next_to_unuse below),
> I recheck under swap_lock whether the entry is charged to the group.
> Even if the device is swapoff'ed, swap_off must have uncharged the entry,
> so I don't think it's needed anyway.
> 
> > YAMAMOTO Takashi
> > 
> > > +			while ((i = find_next_to_unuse(p, i, mem)) != 0) {
> > > +				spin_lock(&swap_lock);
> > > +				if (p->swap_map[i] && p->memcg[i] == mem)
> Ah, I think it should be added !p->swap_map to check the device has not
> been swapoff'ed.

find_next_to_unuse seems to have fragile assumptions and
can dereference p->swap_map as well.

YAMAMOTO Takashi

> 
> 
> Thanks,
> Daisuke Nishimura.
> 
> > > +					swap_cgroup_uncharge(p, i);
> > > +				spin_unlock(&swap_lock);
> > > +			}
> > > +			spin_lock(&swap_lock);
> > > +		}
> > > +	}
> > > +	spin_unlock(&swap_lock);
> > > +
> > > +	return;
> > > +}
> > >  #endif