Re: [PATCH 6/8] mm: lru_deactivate_fn should clear PG_referenced

[Minchan Kim]

On Fri, Oct 30, 2015 at 01:47:11PM +0100, Michal Hocko wrote:
> On Fri 30-10-15 16:01:42, Minchan Kim wrote:
> > deactivate_page aims for accelerate for reclaiming through
> > moving pages from active list to inactive list so we should
> > clear PG_referenced for the goal.
> 
> I might be missing something but aren't we using PG_referenced only for
> pagecache (and shmem) pages?

You don't miss anything. For pages which are candidate of MADV_FREEing(
ie, normal anonymous page, not shmem, tmpfs), they shouldn't have any
PG_referenced. Although normal anonymous pages have it, VM doesn't respect
it. One thing I suspect is GUP with FOLL_TOUCH which calls mark_page_accesssed
on anonymous page and will mark PG_referenced.
Technically, it's not a problem but just want to notice in this time.

Primary reason was I want to make deactivate_page *general* so it could
be used for file page as well as anon pages in future.
But at the moment, user of deactivate_page is only MADV_FREE so it might
be better to merge the logic for anon page deactivation into
deactivate_file_page and rename it as general "deactivate_page"
if you're thinking it's better.

> 
> > 
> > Acked-by: Hugh Dickins <hughd@xxxxxxxxxx>
> > Suggested-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
> > ---
> >  mm/swap.c | 1 +
> >  1 file changed, 1 insertion(+)
> > 
> > diff --git a/mm/swap.c b/mm/swap.c
> > index d0eacc5f62a3..4a6aec976ab1 100644
> > --- a/mm/swap.c
> > +++ b/mm/swap.c
> > @@ -810,6 +810,7 @@ static void lru_deactivate_fn(struct page *page, struct lruvec *lruvec,
> >  
> >  		del_page_from_lru_list(page, lruvec, lru + LRU_ACTIVE);
> >  		ClearPageActive(page);
> > +		ClearPageReferenced(page);
> >  		add_page_to_lru_list(page, lruvec, lru);
> >  
> >  		__count_vm_event(PGDEACTIVATE);
> > -- 
> > 1.9.1
> 
> -- 
> Michal Hocko
> SUSE Labs
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