Re: [PATCH v4 2/5] zswap: make shrinking memcg-aware

[Muchun Song <muchun.song@linux.dev>]

> On Nov 2, 2023, at 01:44, Nhat Pham <nphamcs@gmail.com> wrote:
> 
> On Tue, Oct 31, 2023 at 8:07 PM Muchun Song <muchun.song@linux.dev> wrote:
>> 
>> 
>> 
>>> On Nov 1, 2023, at 09:26, Nhat Pham <nphamcs@gmail.com> wrote:
>>> 
>>> cc-ing Johannes, Roman, Shakeel, Muchun since you all know much more
>>> about memory controller + list_lru reparenting logic than me.
>>> 
>>> There seems to be a race between memcg offlining and zswap’s
>>> cgroup-aware LRU implementation:
>>> 
>>> CPU0                            CPU1
>>> zswap_lru_add()                 mem_cgroup_css_offline()
>>>   get_mem_cgroup_from_objcg()
>>>                                   memcg_offline_kmem()
>>>                                       memcg_reparent_objcgs()
>>>                                       memcg_reparent_list_lrus()
>>>                                           memcg_reparent_list_lru()
>>>                                               memcg_reparent_list_lru_node()
>>>   list_lru_add()
>>>                                               memcg_list_lru_free()
>>> 
>>> 
>>> Essentially: on CPU0, zswap gets the memcg from the entry's objcg
>>> (before the objcgs are reparented). Then it performs list_lru_add()
>>> after the list_lru entries reparenting (memcg_reparent_list_lru_node())
>>> step. If the list_lru of the memcg being offlined has not been freed
>>> (i.e before the memcg_list_lru_free() call), then the list_lru_add()
>>> call would succeed - but the list will be freed soon after. The new
>> 
>> No worries.  list_lru_add() will add the object to the lru list of
>> the parent of the memcg being offlined, because the ->kmemcg_id of the
>> memcg being offlined will be changed to its parent's ->kmemcg_id before memcg_reparent_list_lru().
>> 
> 
> Ohhh that is subtle. Thanks for pointing this out, Muchun!
> 
> In that case, I think Yosry is right after all! We don't even need to get
> a reference to the memcg:
> 
> rcu_read_lock();
> memcg = obj_cgroup_memcg(objcg);
> list_lru_add();
> rcu_read_unlock();
> 
> As long as we're inside this rcu section, we're guaranteed to get
> an un-freed memcg. Now it could be offlined etc., but as Muchun has

Right.

Thanks.

> pointed out, the list_lru_add() call will still does the right thing - it will
> either add the new entry to the parent list if this happens after the
> kmemcg_id update, or the child list before the list_lru reparenting
> action. Both of these scenarios are fine.
> 
>> Muchun,
>> Thanks
>> 
>>> zswap entry as a result will not be subjected to future reclaim
>>> attempt. IOW, this list_lru_add() call is effectively swallowed. And
>>> worse, there might be a crash when we invalidate the zswap_entry in the
>>> future (which will perform a list_lru removal).
>>> 
>>> Within get_mem_cgroup_from_objcg(), none of the following seem
>>> sufficient to prevent this race:
>>> 
>>>   1. Perform the objcg-to-memcg lookup inside a rcu_read_lock()
>>>   section.
>>>   2. Checking if the memcg is freed yet (with css_tryget()) (what
>>>   we're currently doing in this patch series).
>>>   3. Checking if the memcg is still online (with css_tryget_online())
>>>   The memcg can still be offlined down the line.
>>> 
>>> 
>>> I've discussed this privately with Johannes, and it seems like the
>>> cleanest solution here is to move the reparenting logic down to release
>>> stage. That way, when get_mem_cgroup_from_objcg() returns,
>>> zswap_lru_add() is given an memcg that is reparenting-safe (until we
>>> drop the obtained reference).