Re: [PATCH v3] mm: memcg/slab: Stop reparented obj_cgroups from charging root

[Richard Palethorpe]

Hello,

Michal Koutný <mkoutny@xxxxxxxx> writes:

> Hi.
>
> On Tue, Oct 20, 2020 at 06:52:08AM +0100, Richard Palethorpe <rpalethorpe@xxxxxxx> wrote:
>> I don't think that is relevant as we get the memcg from objcg->memcg
>> which is set during reparenting. I suppose however, we can determine if
>> the objcg was reparented by inspecting memcg->objcg.
> +1
>
>> If we just check use_hierarchy then objects directly charged to the
>> memcg where use_hierarchy=0 will not be uncharged. However, maybe it is
>> better to check if it was reparented and if use_hierarchy=0.
> I think (I had to make a table) the yielded condition would be:
>
> if ((memcg->use_hierarchy && reparented) || (!mem_cgroup_is_root(memcg) && !reparented))

This looks correct, but I don't think we need to check for reparenting
after all. We have the following unique scenarious:

use_hierarchy=1, memcg=?, reparented=?:
Because use_hierarchy=1 any descendants will have charged the current
memcg, including root, so we can simply uncharge regardless of the memcg
or objcg.

use_hierarchy=0, memcg!=root, reparented=?:
When use_hierarchy=0, objcgs are *only* reparented to root, so if we are
not root the objcg must belong to us.

use_hierarchy=0, memcg=root, reparented=?:
We must have been reparented because root is not initialised with an
objcg, but use_hierarchy=0 so we can not uncharge.

So I believe that the following is sufficient.

if (memcg->use_hierarchy || !mem_cgroup_is_root(memcg))
> 	 __memcg_kmem_uncharge(memcg, nr_pages);
>
> (I admit it's not very readable.)
>
>
> Michal

For the record, I did create the following patch which checks for
reparenting, but it appears to be unecessary.

----

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 6877c765b8d0..0285f760f003 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -257,6 +257,14 @@ struct cgroup_subsys_state *vmpressure_to_css(struct vmpressure *vmpr)
 #ifdef CONFIG_MEMCG_KMEM
 extern spinlock_t css_set_lock;

+/* Assumes objcg originated from a descendant of memcg or is memcg's */
+static bool obj_cgroup_did_charge(const struct obj_cgroup *objcg,
+                                 const struct mem_cgroup *memcg)
+{
+       return memcg->use_hierarchy ||
+               rcu_access_pointer(memcg->objcg) == objcg;
+}
+
 static void obj_cgroup_release(struct percpu_ref *ref)
 {
        struct obj_cgroup *objcg = container_of(ref, struct obj_cgroup, refcnt);
@@ -291,7 +299,7 @@ static void obj_cgroup_release(struct percpu_ref *ref)

        spin_lock_irqsave(&css_set_lock, flags);
        memcg = obj_cgroup_memcg(objcg);
-       if (nr_pages)
+       if (nr_pages && obj_cgroup_did_charge(objcg, memcg))
                __memcg_kmem_uncharge(memcg, nr_pages);
        list_del(&objcg->list);
        mem_cgroup_put(memcg);
@@ -3100,6 +3108,7 @@ static bool consume_obj_stock(struct obj_cgroup *objcg, unsigned int nr_bytes)
 static void drain_obj_stock(struct memcg_stock_pcp *stock)
 {
        struct obj_cgroup *old = stock->cached_objcg;
+       struct mem_cgroup *memcg;

        if (!old)
                return;
@@ -3110,7 +3119,9 @@ static void drain_obj_stock(struct memcg_stock_pcp *stock)

                if (nr_pages) {
                        rcu_read_lock();
-                       __memcg_kmem_uncharge(obj_cgroup_memcg(old), nr_pages);
+                       memcg = obj_cgroup_memcg(old);
+                       if (obj_cgroup_did_charge(old, memcg))
+                               __memcg_kmem_uncharge(memcg, nr_pages);
                        rcu_read_unlock();
                }

-- 
Thank you,
Richard.