Re: [PATCH v2 2/7] mm: vmscan: make global slab shrink lockless

[Qi Zheng]

On 2023/2/26 00:17, Kirill Tkhai wrote:
> On 25.02.2023 18:57, Qi Zheng wrote:
> >
<...>
> > How about this?
> > > > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > > > index ffddbd204259..9d8c53075298 100644
> > > > --- a/mm/vmscan.c
> > > > +++ b/mm/vmscan.c
> > > > @@ -1012,7 +1012,7 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
> > > >                                    int priority)
> > > >    {
> > > >           unsigned long ret, freed = 0;
> > > > -       struct shrinker *shrinker;
> > > > +       struct shrinker *shrinker = NULL;
> > > >           int srcu_idx, generation;
> > > >
> > > >           /*
> > > > @@ -1025,11 +1025,15 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
> > > >           if (!mem_cgroup_disabled() && !mem_cgroup_is_root(memcg))
> > > >                   return shrink_slab_memcg(gfp_mask, nid, memcg, priority);
> > > >
> > > > +again:
> > > >           srcu_idx = srcu_read_lock(&shrinker_srcu);
> > > >
> > > >           generation = atomic_read(&shrinker_srcu_generation);
> > > > -       list_for_each_entry_srcu(shrinker, &shrinker_list, list,
> > > > -                                srcu_read_lock_held(&shrinker_srcu)) {
> > > > +       if (!shrinker)
> > > > +               shrinker = list_entry_rcu(shrinker_list.next, struct shrinker, list);
> > > > +       else
> > > > +               shrinker = list_entry_rcu(shrinker->list.next, struct shrinker, list);
> > > > +       list_for_each_entry_from_rcu(shrinker, &shrinker_list, list) {
> > > >                   struct shrink_control sc = {
> > > >                           .gfp_mask = gfp_mask,
> > > >                           .nid = nid,
> > > > @@ -1042,8 +1046,9 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
> > > >                   freed += ret;
> > > >
> > > >                   if (atomic_read(&shrinker_srcu_generation) != generation) {
> > > > -                       freed = freed ? : 1;
> > > > -                       break;
> > > > +                       srcu_read_unlock(&shrinker_srcu, srcu_idx);
>
> After SRCU in unlocked we can't believe @shrinker anymore. So, above list_entry_rcu(shrinker->list.next)
> dereferences some random memory.

Indeed.

> > > > +                       cond_resched();
> > > > +                       goto again;
> > > >                   }
> > > >           }
> > > >
> > > > > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > > > > index 27ef9946ae8a..0b197bba1257 100644
> > > > > --- a/mm/vmscan.c
> > > > > +++ b/mm/vmscan.c
> > > > > @@ -204,6 +204,7 @@ static void set_task_reclaim_state(struct task_struct *task,
> > > > >     LIST_HEAD(shrinker_list);
> > > > >     DEFINE_MUTEX(shrinker_mutex);
> > > > >     DEFINE_SRCU(shrinker_srcu);
> > > > > +static atomic_t shrinker_srcu_generation = ATOMIC_INIT(0);
> > > > >       #ifdef CONFIG_MEMCG
> > > > >     static int shrinker_nr_max;
> > > > > @@ -782,6 +783,7 @@ void unregister_shrinker(struct shrinker *shrinker)
> > > > >         debugfs_entry = shrinker_debugfs_remove(shrinker);
> > > > >         mutex_unlock(&shrinker_mutex);
> > > > >     +    atomic_inc(&shrinker_srcu_generation);
> > > > >         synchronize_srcu(&shrinker_srcu);
> > > > >           debugfs_remove_recursive(debugfs_entry);
> > > > > @@ -799,6 +801,7 @@ EXPORT_SYMBOL(unregister_shrinker);
> > > > >      */
> > > > >     void synchronize_shrinkers(void)
> > > > >     {
> > > > > +    atomic_inc(&shrinker_srcu_generation);
> > > > >         synchronize_srcu(&shrinker_srcu);
> > > > >     }
> > > > >     EXPORT_SYMBOL(synchronize_shrinkers);
> > > > > @@ -908,18 +911,19 @@ static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid,
> > > > >     {
> > > > >         struct shrinker_info *info;
> > > > >         unsigned long ret, freed = 0;
> > > > > -    int srcu_idx;
> > > > > -    int i;
> > > > > +    int srcu_idx, generation;
> > > > > +    int i = 0;
> > > > >           if (!mem_cgroup_online(memcg))
> > > > >             return 0;
> > > > > -
> > > > > +again:
> > > > >         srcu_idx = srcu_read_lock(&shrinker_srcu);
> > > > >         info = shrinker_info_srcu(memcg, nid);
> > > > >         if (unlikely(!info))
> > > > >             goto unlock;
> > > > >     -    for_each_set_bit(i, info->map, info->map_nr_max) {
> > > > > +    generation = atomic_read(&shrinker_srcu_generation);
> > > > > +    for_each_set_bit_from(i, info->map, info->map_nr_max) {
> > > > >             struct shrink_control sc = {
> > > > >                 .gfp_mask = gfp_mask,
> > > > >                 .nid = nid,
> > > > > @@ -965,6 +969,11 @@ static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid,
> > > > >                     set_shrinker_bit(memcg, nid, i);
> > > > >             }
> > > > >             freed += ret;
> > > > > +
> > > > > +        if (atomic_read(&shrinker_srcu_generation) != generation) {
> > > > > +            srcu_read_unlock(&shrinker_srcu, srcu_idx);
> > > >
> > > > Maybe we can add the following code here, so as to avoid repeating the
> > > > current id and avoid triggering softlockup:
> > > >
> > > >               i++;
>
> This is OK.
>
> > > >               cond_resched();
>
> Possible, existing cond_resched() in do_shrink_slab() is enough.

Yeah.

I will add this patch in the next version. May I mark you as the author
of this patch?

Thanks,
Qi

> > And this. :)
> >
> > Thanks,
> > Qi
> >
> > > > Thanks,
> > > > Qi
> > > >
> > > > > +            goto again;
> > > > > +        }
> > > > >         }
> > > > >     unlock:
> > > > >         srcu_read_unlock(&shrinker_srcu, srcu_idx);
> > > > > @@ -1004,7 +1013,7 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
> > > > >     {
> > > > >         unsigned long ret, freed = 0;
> > > > >         struct shrinker *shrinker;
> > > > > -    int srcu_idx;
> > > > > +    int srcu_idx, generation;
> > > > >           /*
> > > > >          * The root memcg might be allocated even though memcg is disabled
> > > > > @@ -1017,6 +1026,7 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
> > > > >             return shrink_slab_memcg(gfp_mask, nid, memcg, priority);
> > > > >           srcu_idx = srcu_read_lock(&shrinker_srcu);
> > > > > +    generation = atomic_read(&shrinker_srcu_generation);
> > > > >           list_for_each_entry_srcu(shrinker, &shrinker_list, list,
> > > > >                      srcu_read_lock_held(&shrinker_srcu)) {
> > > > > @@ -1030,6 +1040,11 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
> > > > >             if (ret == SHRINK_EMPTY)
> > > > >                 ret = 0;
> > > > >             freed += ret;
> > > > > +
> > > > > +        if (atomic_read(&shrinker_srcu_generation) != generation) {
> > > > > +            freed = freed ? : 1;
> > > > > +            break;
> > > > > +        }
> > > > >         }
> > > > >           srcu_read_unlock(&shrinker_srcu, srcu_idx);

--
Thanks,
Qi