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

[Muchun Song <songmuchun@bytedance.com>]

On Thu, Jan 13, 2022 at 9:32 PM Michal Koutný <mkoutny@suse.com> wrote:
>
> On Wed, Jan 12, 2022 at 09:22:36PM +0800, Muchun Song <songmuchun@bytedance.com> 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?
>

I have thought about this. It's a little different to rely on objcg
reparenting since the user can get memcg from objcg and
then does not realize the memcg has reparented. Although it
can check memcg->objcg to know whether the memcg is
reparented, it should also prevent this memcg from being
reparented throughout memcg_list_lru_alloc(). Maybe
holding css_set_lock can do that. I do not think this
is a good choice. Do you have any thoughts about this?

Thanks.