Re: [PATCH v5 10/16] mm: list_lru: allocate list_lru_one only when needed

[Michal Koutný]

On Wed, Jan 12, 2022 at 09:22:36PM +0800, Muchun Song <songmuchun@xxxxxxxxxxxxx> wrote:
>   root(-1) -> A(0) -> B(1) -> C(2)
> 
> CPU0:                                   CPU1:
> memcg_list_lru_alloc(C)
>                                         memcg_drain_all_list_lrus(C)
>                                         memcg_drain_all_list_lrus(B)
>                                         // Now C and B are offline. The
>                                         // kmemcg_id becomes the following if
>                                         // we do not the kmemcg_id of its
>                                         // descendants in
>                                         // memcg_drain_all_list_lrus().
>                                         //
>                                         // root(-1) -> A(0) -> B(0) -> C(1)
> 
>   for (i = 0; memcg; memcg = parent_mem_cgroup(memcg), i++) {
>       // allocate struct list_lru_per_memcg for memcg C
>       table[i].mlru = memcg_init_list_lru_one(gfp);
>   }
> 
>   spin_lock_irqsave(&lru->lock, flags);
>   while (i--) {
>       // here index = 1
>       int index = table[i].memcg->kmemcg_id;
> 
>       struct list_lru_per_memcg *mlru = table[i].mlru;
>       if (index < 0 || rcu_dereference_protected(mlrus->mlru[index], true))
>           kfree(mlru);
>       else
>           // mlrus->mlru[index] will be assigned a new value regardless
>           // memcg C is already offline.
>           rcu_assign_pointer(mlrus->mlru[index], mlru);
>   }
>   spin_unlock_irqrestore(&lru->lock, flags);
> 

> So changing ->kmemcg_id of all its descendants can prevent
> memcg_list_lru_alloc() from allocating list lrus for the offlined
> cgroup after memcg_list_lru_free() calling.

Thanks for the illustrative example. I can see how this can be a problem
in a general call of memcg_list_lru_alloc(C).

However, the code, as I understand it, resolves the memcg to which lru
allocation should be associated via get_mem_cgroup_from_objcg() and
memcg_reparent_list_lrus(C) comes after memcg_reparent_objcgs(C, B),
i.e. the allocation would target B (or even A if after
memcg_reparent_objcgs(B, A))?

It seems to me like "wasting" the existing objcg reparenting mechanism.
Or what do you think could be a problem relying on it?

Thanks,
Michal