On Thu, Jan 13, 2022 at 9:32 PM Michal Koutný <mkoutny@xxxxxxxx> wrote:
>
> 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?
>
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.
