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

[Michal Hocko]

On Thu 05-12-19 13:00:31, 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.

OK, I see. The changelog should describe all that. Thanks for the
clarification.

> > 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
> >>>
> > 
> 

-- 
Michal Hocko
SUSE Labs