Re: [PATCH] zsmalloc: consider ZS_ALMOST_FULL as migrate source

[Minchan Kim <minchan@kernel.org>]

Hi Sergey,

On Fri, Jul 10, 2015 at 10:58:28AM +0900, Sergey Senozhatsky wrote:
> On (07/10/15 10:32), Minchan Kim wrote:
> > There is no reason to prevent select ZS_ALMOST_FULL as migration
> > source if we cannot find source from ZS_ALMOST_EMPTY.
> > 
> > With this patch, zs_can_compact will return more exact result.
> > 
> 
> wouldn't that be too aggresive?
> 
> drainig 'only ZS_ALMOST_EMPTY classes' sounds safer than draining
> 'ZS_ALMOST_EMPTY and ZS_ALMOST_FULL clases'. you seemed to be worried
> that compaction can leave no unused objects in classes, which will
> result in zspage allocation happening right after compaction. it looks
> like here the chances to cause zspage allocation are even higher. don't
> you think so?

Good question.

My worry was failure of order-0 page allocation in zram-swap path
when memory presssure is really heavy but I didn't insist to you
from sometime. The reason I changed my mind was

1. It's almost dead system if there is no order-0 page
2. If old might be working well, it's not our design, just luck.


> 
> > Signed-off-by: Minchan Kim <minchan@kernel.org>
> > ---
> >  mm/zsmalloc.c |   19 ++++++++++++-------
> >  1 file changed, 12 insertions(+), 7 deletions(-)
> > 
> > diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> > index 8c78bcb..7bd7dde 100644
> > --- a/mm/zsmalloc.c
> > +++ b/mm/zsmalloc.c
> > @@ -1687,12 +1687,20 @@ static enum fullness_group putback_zspage(struct zs_pool *pool,
> >  static struct page *isolate_source_page(struct size_class *class)
> >  {
> >  	struct page *page;
> > +	int i;
> > +	bool found = false;
> >  
> > -	page = class->fullness_list[ZS_ALMOST_EMPTY];
> > -	if (page)
> > -		remove_zspage(page, class, ZS_ALMOST_EMPTY);
> > +	for (i = ZS_ALMOST_EMPTY; i >= ZS_ALMOST_FULL; i--) {
> > +		page = class->fullness_list[i];
> > +		if (!page)
> > +			continue;
> >  
> > -	return page;
> > +		remove_zspage(page, class, i);
> > +		found = true;
> > +		break;
> > +	}
> > +
> > +	return found ? page : NULL;
> >  }
> >  
> >  /*
> > @@ -1706,9 +1714,6 @@ static unsigned long zs_can_compact(struct size_class *class)
> >  {
> >  	unsigned long obj_wasted;
> >  
> > -	if (!zs_stat_get(class, CLASS_ALMOST_EMPTY))
> > -		return 0;
> > -
> 
> well, you asked to add this check like a week or two ago (it's not even
> in -next yet) and now you remove it.

The reason I wanted to check CLASS_ALMOST_EMPTY is to make zs_can_compact exact.
But with this patch, we can achieve it without above.

> 
> >  	obj_wasted = zs_stat_get(class, OBJ_ALLOCATED) -
> >  		zs_stat_get(class, OBJ_USED);
> >  
> 
> 	-ss

-- 
Kind regards,
Minchan Kim

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