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

[Daisuke Nishimura]

Hi, Yamamoto-san.

Thank you for your comment.

On Fri,  4 Jul 2008 15:54:31 +0900 (JST), yamamoto at valinux.co.jp (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.


Thanks,
Daisuke Nishimura.

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