Re: [v2 PATCH] mm: vmscan: correct nr_reclaimed for THP

[Yang Shi]

On 5/13/19 1:09 AM, Michal Hocko wrote:
> On Sat 11-05-19 00:23:40, Yang Shi wrote:
> > 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.
> You are technically right here. This has been a known issue for a while.
> I am wondering whether somebody actually noticed some misbehavior.
> Swapping out is a rare event and you have to have a considerable number
> of THPs to notice.

The commit bd4c82c22c36 ("mm, THP, swap: delay splitting THP after 
swapped out") was added in 4.14, it might be not used widely yet. And we 
need swap + THPs to trigger it, furthermore the misbehavior might be 
accumulative over time. I don't expect it will have any obvious 
misbehavior for a single shot.

> > 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.
> This is quite nasty and confusing. I am worried that having those two
> unsynced begs for subtle issues. Can we account THP as scanning 512 base
> pages as well?

Actually, such unsync has been there for a while. The 
isolate_lru_pages() returns nr_taken which has THP accounted as 512 base 
pages, but nr_scanned just shows 1. nr_taken is not showed via 
/proc/vmstat, but it is showed via trace point.

I think we can just account 512 base pages for nr_scanned for 
isolate_lru_pages() to make the counters sane since PGSCAN_KSWAPD/DIRECT 
just use it.

And, sc->nr_scanned should be accounted as 512 base pages too otherwise 
we may have nr_scanned < nr_to_reclaim all the time to result in 
false-negative for priority raise and something else wrong (e.g. wrong 
vmpressure).

> > Cc: "Huang, Ying" <ying.huang@xxxxxxxxx>
> > Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> > Cc: Michal Hocko <mhocko@xxxxxxxx>
> > Cc: Mel Gorman <mgorman@xxxxxxx>
> > Cc: "Kirill A . Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
> > Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> > Reviewed-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
> > Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
> > ---
> > v2: Added Shakeel's Reviewed-by
> >      Use hpage_nr_pages instead of compound_order per Huang Ying and William Kucharski
> >
> >   mm/vmscan.c | 6 +++++-
> >   1 file changed, 5 insertions(+), 1 deletion(-)
> >
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index fd9de50..4226d6b 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 += hpage_nr_pages(page);
> >   
> >   		/*
> >   		 * Is there need to periodically free_page_list? It would
> > --
> > 1.8.3.1