Re: [PATCH] mm/memcg: Fix incorrect flushing of lruvec data in obj_stock

[Waiman Long]

On 8/2/21 2:28 AM, Michal Hocko wrote:
> On Sun 01-08-21 22:28:27, Waiman Long wrote:
> > When mod_objcg_state() is called with a pgdat that is different from
> > that in the obj_stock, the old lruvec data cached in obj_stock are
> > flushed out. Unfortunately, they were flushed to the new pgdat and
> > hence the wrong node, not the one cached in obj_stock.
> It would be great to explicitly mention user observable problems here. I
> do assume this will make slab stats skewed but the effect wouldn't be
> very big, right?
It is the /sys/devices/system/node/node*/meminfo that will get skewed. 
Not /proc/meminfo. So it is a relatively minor issue. Will update the 
patch to mention that.
> > Fix that by flushing the data to the cached pgdat instead.
> >
> > Fixes: 68ac5b3c8db2 ("mm/memcg: cache vmstat data in percpu memcg_stock_pcp")
> > Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
> Acked-by: Michal Hocko <mhocko@xxxxxxxx>
>
> > ---
> >   mm/memcontrol.c | 8 +++++---
> >   1 file changed, 5 insertions(+), 3 deletions(-)
> >
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index ae1f5d0cb581..881ec4ddddcd 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -3106,17 +3106,19 @@ void mod_objcg_state(struct obj_cgroup *objcg, struct pglist_data *pgdat,
> >   		stock->cached_pgdat = pgdat;
> >   	} else if (stock->cached_pgdat != pgdat) {
> >   		/* Flush the existing cached vmstat data */
> > +		struct pglist_data *oldpg = stock->cached_pgdat;
> > +
> > +		stock->cached_pgdat = pgdat;
> >   		if (stock->nr_slab_reclaimable_b) {
> > -			mod_objcg_mlstate(objcg, pgdat, NR_SLAB_RECLAIMABLE_B,
> > +			mod_objcg_mlstate(objcg, oldpg, NR_SLAB_RECLAIMABLE_B,
> >   					  stock->nr_slab_reclaimable_b);
> >   			stock->nr_slab_reclaimable_b = 0;
> >   		}
> >   		if (stock->nr_slab_unreclaimable_b) {
> > -			mod_objcg_mlstate(objcg, pgdat, NR_SLAB_UNRECLAIMABLE_B,
> > +			mod_objcg_mlstate(objcg, oldpg, NR_SLAB_UNRECLAIMABLE_B,
> >   					  stock->nr_slab_unreclaimable_b);
> >   			stock->nr_slab_unreclaimable_b = 0;
> >   		}
> > -		stock->cached_pgdat = pgdat;
> Minor nit. Is there any reason to move the cached_pgdat? TBH I found the
> original way better from the readability POV.

Right. Will move it back to its original place.

Cheers,
Longman