Re: [PATCH v2] mm/vmscan: shrink slab in node reclaim

[Mel Gorman <mgorman@techsingularity.net>]

On Tue, Aug 06, 2019 at 05:32:54PM +0800, Yafang Shao wrote:
> On Tue, Aug 6, 2019 at 5:25 PM Michal Hocko <mhocko@kernel.org> wrote:
> >
> > On Tue 06-08-19 17:15:05, Yafang Shao wrote:
> > > On Tue, Aug 6, 2019 at 5:05 PM Michal Hocko <mhocko@kernel.org> wrote:
> > [...]
> > > > > As you said, the direct reclaim path set it to 1, but the
> > > > > __node_reclaim() forgot to process may_shrink_slab.
> > > >
> > > > OK, I am blind obviously. Sorry about that. Anyway, why cannot we simply
> > > > get back to the original behavior by setting may_shrink_slab in that
> > > > path as well?
> > >
> > > You mean do it as the commit 0ff38490c836 did  before ?
> > > I haven't check in which commit the shrink_slab() is removed from
> >
> > What I've had in mind was essentially this:
> >
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 7889f583ced9..8011288a80e2 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -4088,6 +4093,7 @@ static int __node_reclaim(struct pglist_data *pgdat, gfp_t gfp_mask, unsigned in
> >                 .may_unmap = !!(node_reclaim_mode & RECLAIM_UNMAP),
> >                 .may_swap = 1,
> >                 .reclaim_idx = gfp_zone(gfp_mask),
> > +               .may_shrinkslab = 1;
> >         };
> >
> >         trace_mm_vmscan_node_reclaim_begin(pgdat->node_id, order,
> >
> > shrink_node path already does shrink slab when the flag allows that. In
> > other words get us back to before 1c30844d2dfe because that has clearly
> > changed the long term node reclaim behavior just recently.
> > --
> 
> If we do it like this, then vm.min_slab_ratio will not take effect if
> there're enough relcaimable page cache.
> Seems there're bugs in the original behavior as well.
> 

Typically that would be done as a separate patch with a standalone
justification for it. The first patch should simply restore expected
behaviour with a Fixes: tag noting that the change in behaviour was
unintentional.

-- 
Mel Gorman
SUSE Labs