Re: [RFC PATCH 20/26] mm: vmscan: use compaction_suitable() check in kswapd

[Johannes Weiner <hannes@cmpxchg.org>]

On Tue, Apr 25, 2023 at 11:12:28AM +0800, Huang, Ying wrote:
> Johannes Weiner <hannes@cmpxchg.org> writes:
> 
> > Kswapd currently bails on higher-order allocations with an open-coded
> > check for whether it's reclaimed the compaction gap.
> >
> > compaction_suitable() is the customary interface to coordinate reclaim
> > with compaction.
> >
> > Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> > ---
> >  mm/vmscan.c | 67 ++++++++++++++++++-----------------------------------
> >  1 file changed, 23 insertions(+), 44 deletions(-)
> >
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index ee8c8ca2e7b5..723705b9e4d9 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -6872,12 +6872,18 @@ static bool pgdat_balanced(pg_data_t *pgdat, int order, int highest_zoneidx)
> >  		if (!managed_zone(zone))
> >  			continue;
> >  
> > +		/* Allocation can succeed in any zone, done */
> >  		if (sysctl_numa_balancing_mode & NUMA_BALANCING_MEMORY_TIERING)
> >  			mark = wmark_pages(zone, WMARK_PROMO);
> >  		else
> >  			mark = high_wmark_pages(zone);
> >  		if (zone_watermark_ok_safe(zone, order, mark, highest_zoneidx))
> >  			return true;
> > +
> > +		/* Allocation can't succeed, but enough order-0 to compact */
> > +		if (compaction_suitable(zone, order,
> > +					highest_zoneidx) == COMPACT_CONTINUE)
> > +			return true;
> 
> Should we check the following first?
> 
>         order > 0 && zone_watermark_ok_safe(zone, 0, mark, highest_zoneidx)

That's what compaction_suitable() does. It checks whether there are
enough migration targets for compaction (COMPACT_CONTINUE) or whether
reclaim needs to do some more work (COMPACT_SKIPPED).