Re: [rfc 1/3] mm: vmscan: never swap under low memory pressure

[Johannes Weiner <jweiner@redhat.com>]

On Mon, Nov 07, 2011 at 11:29:41AM +0900, KAMEZAWA Hiroyuki wrote:
> On Wed, 2 Nov 2011 17:31:41 +0100
> Johannes Weiner <jweiner@redhat.com> wrote:
> 
> > We want to prevent floods of used-once file cache pushing us to swap
> > out anonymous pages.  Never swap under a certain priority level.  The
> > availability of used-once cache pages should prevent us from reaching
> > that threshold.
> > 
> > This is needed because subsequent patches will revert some of the
> > mechanisms that tried to prefer file over anon, and this should not
> > result in more eager swapping again.
> > 
> > It might also be better to keep the aging machinery going and just not
> > swap, rather than staying away from anonymous pages in the first place
> > and having less useful age information at the time of swapout.
> > 
> > Signed-off-by: Johannes Weiner <jweiner@redhat.com>
> > ---
> >  mm/vmscan.c |    2 ++
> >  1 files changed, 2 insertions(+), 0 deletions(-)
> > 
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index a90c603..39d3da3 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -831,6 +831,8 @@ static unsigned long shrink_page_list(struct list_head *page_list,
> >  		 * Try to allocate it some swap space here.
> >  		 */
> >  		if (PageAnon(page) && !PageSwapCache(page)) {
> > +			if (priority >= DEF_PRIORITY - 2)
> > +				goto keep_locked;
> >  			if (!(sc->gfp_mask & __GFP_IO))
> >  				goto keep_locked;
> >  			if (!add_to_swap(page))
> 
> Hm, how about not scanning LRU_ANON rather than checking here ?
> Add some bias to get_scan_count() or some..
> If you think to need rotation of LRU, only kswapd should do that..

Absolutely, it would require more tuning.  This patch was really a
'hey, how about we do something like this?  anyone tried that before?'

I keep those things in mind if I pursue this further, thanks.