Re: [PATCH] mm: vmscan: protect shrinker idr replace with CONFIG_MEMCG

[Yang Shi]

On 12/5/19 2:11 AM, Kirill Tkhai wrote:
> On 05.12.2019 13:00, Kirill Tkhai wrote:
> > On 05.12.2019 12:43, Michal Hocko wrote:
> > > On Thu 05-12-19 11:23:28, Kirill Tkhai wrote:
> > > > On 04.12.2019 22:16, Yang Shi wrote:
> > > > > Since commit 0a432dcbeb32edcd211a5d8f7847d0da7642a8b4 ("mm: shrinker:
> > > > > make shrinker not depend on memcg kmem"), shrinkers' idr is protected by
> > > > > CONFIG_MEMCG instead of CONFIG_MEMCG_KMEM, so it makes no sense to
> > > > > protect shrinker idr replace with CONFIG_MEMCG_KMEM.
> > > > >
> > > > > Cc: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>
> > > > > Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> > > > > Cc: Michal Hocko <mhocko@xxxxxxxx>
> > > > > Cc: Shakeel Butt <shakeelb@xxxxxxxxxx>
> > > > > Cc: Roman Gushchin <guro@xxxxxx>
> > > > > Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
> > > > It looks like that in CONFIG_SLOB case we do not even call some shrinkers
> > > > for subordinate mem cgroups (i.e., we don't call deferred_split_shrinker),
> > > > since they never become completely registered.
> > > >
> > > > Fixes: 0a432dcbeb32edcd211a5d8f7847d0da7642a8b4 ("mm: shrinker: make shrinker not depend on memcg kmem")
> > > I am confused. Why the Fixes tag? Nothing should be really broken with
> > > KMEM config guard right?
> > idr_replace() is disabled in CONFIG_MEMCG && CONFIG_SLOB case, and this is
> > wrong.
> >
> > 0a432dcbeb32edcd211a5d8f7847d0da7642a8b4 goes in the series, which enables
> > shrinker_idr infrastructure for huge_memory.c's deferred_split_shrinker
> > in CONFIG_MEMCG case. Previously, all SHRINKER_MEMCG_AWARE shrinkers were
> > based on LRUs, and they remain to base of CONFIG_MEMCG_KMEM.
> > But deferred_split_shrinker is an exception.
> >
> > In CONFIG_MEMCG && CONFIG_SLOB case, shrinker_idr contains only shrinker,
> > and it is deferred_split_shrinker. But it is never actually called, since
> > idr_replace() is never compiled. deferred_split_shrinker all the time is
> > staying in half-registered state, and it's never called for subordinate
> > mem cgroups.
> >
> > So, this is a BUG, and this should go to stable.
> The visible effect is that deferred_split_shrinker is never called
> from shrink_slab() for subordinate mem cgroups. It's called only
> for root_mem_cgroup.

Thanks for noticing the SLOB case, I didn't realize it and thought it 
was just a cleanup too.

Will update the information and cc to stable list for v2.

> > > This is a mere clean up AFAICS.
> > >
> > > > Reviewed-by: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>
> > > >
> > > > > ---
> > > > >   mm/vmscan.c | 2 +-
> > > > >   1 file changed, 1 insertion(+), 1 deletion(-)
> > > > >
> > > > > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > > > > index ee4eecc..e7f10c4 100644
> > > > > --- a/mm/vmscan.c
> > > > > +++ b/mm/vmscan.c
> > > > > @@ -422,7 +422,7 @@ void register_shrinker_prepared(struct shrinker *shrinker)
> > > > >   {
> > > > >   	down_write(&shrinker_rwsem);
> > > > >   	list_add_tail(&shrinker->list, &shrinker_list);
> > > > > -#ifdef CONFIG_MEMCG_KMEM
> > > > > +#ifdef CONFIG_MEMCG
> > > > >   	if (shrinker->flags & SHRINKER_MEMCG_AWARE)
> > > > >   		idr_replace(&shrinker_idr, shrinker, shrinker->id);
> > > > >   #endif