Re: [PATCH v4 5/7] mm: rework non-root kmem_cache lifecycle management

[Waiman Long <longman@redhat.com>]

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On 5/14/19 8:06 PM, Shakeel Butt wrote:
>> @@ -2651,20 +2652,35 @@ struct kmem_cache *memcg_kmem_get_cache(struct kmem_cache *cachep)
>>         struct mem_cgroup *memcg;
>>         struct kmem_cache *memcg_cachep;
>>         int kmemcg_id;
>> +       struct memcg_cache_array *arr;
>>
>>         VM_BUG_ON(!is_root_cache(cachep));
>>
>>         if (memcg_kmem_bypass())
>>                 return cachep;
>>
>> -       memcg = get_mem_cgroup_from_current();
>> +       rcu_read_lock();
>> +
>> +       if (unlikely(current->active_memcg))
>> +               memcg = current->active_memcg;
>> +       else
>> +               memcg = mem_cgroup_from_task(current);
>> +
>> +       if (!memcg || memcg == root_mem_cgroup)
>> +               goto out_unlock;
>> +
>>         kmemcg_id = READ_ONCE(memcg->kmemcg_id);
>>         if (kmemcg_id < 0)
>> -               goto out;
>> +               goto out_unlock;
>>
>> -       memcg_cachep = cache_from_memcg_idx(cachep, kmemcg_id);
>> -       if (likely(memcg_cachep))
>> -               return memcg_cachep;
>> +       arr = rcu_dereference(cachep->memcg_params.memcg_caches);
>> +
>> +       /*
>> +        * Make sure we will access the up-to-date value. The code updating
>> +        * memcg_caches issues a write barrier to match this (see
>> +        * memcg_create_kmem_cache()).
>> +        */
>> +       memcg_cachep = READ_ONCE(arr->entries[kmemcg_id]);
>>
>>         /*
>>          * If we are in a safe context (can wait, and not in interrupt
>> @@ -2677,10 +2693,20 @@ struct kmem_cache *memcg_kmem_get_cache(struct kmem_cache *cachep)
>>          * memcg_create_kmem_cache, this means no further allocation
>>          * could happen with the slab_mutex held. So it's better to
>>          * defer everything.
>> +        *
>> +        * If the memcg is dying or memcg_cache is about to be released,
>> +        * don't bother creating new kmem_caches. Because memcg_cachep
>> +        * is ZEROed as the fist step of kmem offlining, we don't need
>> +        * percpu_ref_tryget() here. css_tryget_online() check in
> *percpu_ref_tryget_live()
>
>> +        * memcg_schedule_kmem_cache_create() will prevent us from
>> +        * creation of a new kmem_cache.
>>          */
>> -       memcg_schedule_kmem_cache_create(memcg, cachep);
>> -out:
>> -       css_put(&memcg->css);
>> +       if (unlikely(!memcg_cachep))
>> +               memcg_schedule_kmem_cache_create(memcg, cachep);
>> +       else if (percpu_ref_tryget(&memcg_cachep->memcg_params.refcnt))
>> +               cachep = memcg_cachep;
>> +out_unlock:
>> +       rcu_read_lock();

There is one more bug that causes the kernel to panic on bootup when I
turned on debugging options.

[   49.871437] =============================
[   49.875452] WARNING: suspicious RCU usage
[   49.879476] 5.2.0-rc1.bz1699202_memcg_test+ #2 Not tainted
[   49.884967] -----------------------------
[   49.888991] include/linux/rcupdate.h:268 Illegal context switch in
RCU read-side critical section!
[   49.897950]
[   49.897950] other info that might help us debug this:
[   49.897950]
[   49.905958]
[   49.905958] rcu_scheduler_active = 2, debug_locks = 1
[   49.912492] 3 locks held by systemd/1:
[   49.916252]  #0: 00000000633673c5 (&type->i_mutex_dir_key#5){.+.+},
at: lookup_slow+0x42/0x70
[   49.924788]  #1: 0000000029fa8c75 (rcu_read_lock){....}, at:
memcg_kmem_get_cache+0x12b/0x910
[   49.933316]  #2: 0000000029fa8c75 (rcu_read_lock){....}, at:
memcg_kmem_get_cache+0x3da/0x910

It should be "rcu_read_unlock();" at the end.

-Longman


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