Re: [PATCH] mm: vmscan: correct nr_reclaimed for THP

[Yang Shi]

On 5/9/19 8:03 PM, Huang, Ying wrote:
> Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx> writes:
>
> > On 5/9/19 7:12 PM, Huang, Ying wrote:
> > > Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx> writes:
> > >
> > > > Since commit bd4c82c22c36 ("mm, THP, swap: delay splitting THP after
> > > > swapped out"), THP can be swapped out in a whole.  But, nr_reclaimed
> > > > still gets inc'ed by one even though a whole THP (512 pages) gets
> > > > swapped out.
> > > >
> > > > This doesn't make too much sense to memory reclaim.  For example, direct
> > > > reclaim may just need reclaim SWAP_CLUSTER_MAX pages, reclaiming one THP
> > > > could fulfill it.  But, if nr_reclaimed is not increased correctly,
> > > > direct reclaim may just waste time to reclaim more pages,
> > > > SWAP_CLUSTER_MAX * 512 pages in worst case.
> > > >
> > > > This change may result in more reclaimed pages than scanned pages showed
> > > > by /proc/vmstat since scanning one head page would reclaim 512 base pages.
> > > >
> > > > Cc: "Huang, Ying" <ying.huang@xxxxxxxxx>
> > > > Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> > > > Cc: Michal Hocko <mhocko@xxxxxxxx>
> > > > Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> > > > Cc: "Kirill A . Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
> > > > Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> > > > Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
> > > > ---
> > > > I'm not quite sure if it was the intended behavior or just omission. I tried
> > > > to dig into the review history, but didn't find any clue. I may miss some
> > > > discussion.
> > > >
> > > >    mm/vmscan.c | 6 +++++-
> > > >    1 file changed, 5 insertions(+), 1 deletion(-)
> > > >
> > > > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > > > index fd9de50..7e026ec 100644
> > > > --- a/mm/vmscan.c
> > > > +++ b/mm/vmscan.c
> > > > @@ -1446,7 +1446,11 @@ static unsigned long shrink_page_list(struct list_head *page_list,
> > > >      		unlock_page(page);
> > > >    free_it:
> > > > -		nr_reclaimed++;
> > > > +		/*
> > > > +		 * THP may get swapped out in a whole, need account
> > > > +		 * all base pages.
> > > > +		 */
> > > > +		nr_reclaimed += (1 << compound_order(page));
> > > >      		/*
> > > >    		 * Is there need to periodically free_page_list? It would
> > > Good catch!  Thanks!
> > >
> > > How about to change this to
> > >
> > >
> > >           nr_reclaimed += hpage_nr_pages(page);
> > Either is fine to me. Is this faster than "1 << compound_order(page)"?
> I think the readability is a little better.  And this will become
>
>          nr_reclaimed += 1
>
> if CONFIG_TRANSPARENT_HUAGEPAGE is disabled.

Good point. Will update in v2 soon.

> Best Regards,
> Huang, Ying
>
> > > Best Regards,
> > > Huang, Ying