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

[Yafang Shao <laoar.shao@gmail.com>]

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.

Thanks
Yafang