Re: [PATCH 3/4] mm: move lazy free pages to inactive list

[Minchan Kim <minchan@kernel.org>]

Hello Andrew,

On Fri, Mar 20, 2015 at 03:43:58PM -0700, Andrew Morton wrote:
> On Wed, 11 Mar 2015 10:20:37 +0900 Minchan Kim <minchan@kernel.org> wrote:
> 
> > MADV_FREE is hint that it's okay to discard pages if there is
> > memory pressure and we uses reclaimers(ie, kswapd and direct reclaim)
> > to free them so there is no worth to remain them in active anonymous LRU
> > so this patch moves them to inactive LRU list's head.
> > 
> > This means that MADV_FREE-ed pages which were living on the inactive list
> > are reclaimed first because they are more likely to be cold rather than
> > recently active pages.
> > 
> > A arguable issue for the approach would be whether we should put it to
> > head or tail in inactive list. I selected *head* because kernel cannot
> > make sure it's really cold or warm for every MADV_FREE usecase but
> > at least we know it's not *hot* so landing of inactive head would be
> > comprimise for various usecases.
> > 
> > This is fixing a suboptimal behavior of MADV_FREE when pages living on
> > the active list will sit there for a long time even under memory
> > pressure while the inactive list is reclaimed heavily. This basically
> > breaks the whole purpose of using MADV_FREE to help the system to free
> > memory which is might not be used.
> > 
> > @@ -789,6 +790,23 @@ static void lru_deactivate_file_fn(struct page *page, struct lruvec *lruvec,
> >  	update_page_reclaim_stat(lruvec, file, 0);
> >  }
> >  
> > +
> > +static void lru_deactivate_fn(struct page *page, struct lruvec *lruvec,
> > +			    void *arg)
> >
> > ...
> >
> > @@ -844,6 +866,18 @@ void deactivate_file_page(struct page *page)
> >  	}
> >  }
> >  
> > +void deactivate_page(struct page *page)
> > +{
> 
> lru_deactivate_file_fn() and deactivate_file_page() are carefully
> documented and lru_deactivate_fn() and deactivate_page() should
> be as well.  In fact it becomes more important now that we have two
> similar-looking things.

Sorry, I have missed this comment.

Acutally, deactive_file_page was too specific on file-backed page
invalidation when I implemented first time. That's why it had a lot
description but deactivate_page is too general so I think short comment
is enough. :)

Here it goes.

Thanks.