Re: [PATCH 6/7] mm: free swap cache aggressively if memcg swap is full

[Vladimir Davydov]

On Fri, Dec 11, 2015 at 02:33:58PM -0500, Johannes Weiner wrote:
> On Thu, Dec 10, 2015 at 02:39:19PM +0300, Vladimir Davydov wrote:
> > Swap cache pages are freed aggressively if swap is nearly full (>50%
> > currently), because otherwise we are likely to stop scanning anonymous
> > when we near the swap limit even if there is plenty of freeable swap
> > cache pages. We should follow the same trend in case of memory cgroup,
> > which has its own swap limit.
> > 
> > Signed-off-by: Vladimir Davydov <vdavydov@xxxxxxxxxxxxx>
> 
> Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> 
> One note:
> 
> > @@ -5839,6 +5839,29 @@ long mem_cgroup_get_nr_swap_pages(struct mem_cgroup *memcg)
> >  	return nr_swap_pages;
> >  }
> >  
> > +bool mem_cgroup_swap_full(struct page *page)
> > +{
> > +	struct mem_cgroup *memcg;
> > +
> > +	VM_BUG_ON_PAGE(!PageLocked(page), page);
> > +
> > +	if (vm_swap_full())
> > +		return true;
> > +	if (!do_swap_account || !PageSwapCache(page))
> > +		return false;
> 
> The callers establish PageSwapCache() under the page lock, which makes
> sense since they only inquire about the swap state when deciding what
> to do with a swapcache page at hand. So this check seems unnecessary.

Yeah, you're right, we don't need it here. Will remove it in v2.

Besides, I think I should have inserted cgroup_subsys_on_dflt check in
this function so that it wouldn't check memcg->swap limit in case the
legacy hierarchy is used. Will do.

Thanks,
Vladimir
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