On Fri, 14 Jun 2013 09:54:57 +0800 Li Zefan <lizefan@xxxxxxxxxx> wrote:
> Use css_get/put instead of mem_cgroup_get/put.
>
> We can't do a simple replacement, because here mem_cgroup_put()
> is called during mem_cgroup_css_free(), while mem_cgroup_css_free()
> won't be called until css refcnt goes down to 0.
>
> Instead we increment css refcnt in mem_cgroup_css_offline(), and
> then check if there's still kmem charges. If not, css refcnt will
> be decremented immediately, otherwise the refcnt will be released
> after the last kmem allocation is uncahred.
>
> ...
>
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -416,6 +416,11 @@ static void memcg_kmem_clear_activated(struct mem_cgroup *memcg)
>
> static void memcg_kmem_mark_dead(struct mem_cgroup *memcg)
> {
> + /*
> + * We need to call css_get() first, because memcg_uncharge_kmem()
> + * will call css_put() if it sees the memcg is dead.
> + */
> + smp_wmb();
> if (test_bit(KMEM_ACCOUNTED_ACTIVE, &memcg->kmem_account_flags))
> set_bit(KMEM_ACCOUNTED_DEAD, &memcg->kmem_account_flags);
> }
That comment is rather confusing and unhelpful. "We need to call
css_get", but we clearly *don't* call css_get(). I guess we want
--- a/mm/memcontrol.c~memcg-use-css_get-put-when-charging-uncharging-kmem-fix
+++ a/mm/memcontrol.c
@@ -407,7 +407,7 @@ static void memcg_kmem_clear_activated(s
static void memcg_kmem_mark_dead(struct mem_cgroup *memcg)
{
/*
- * We need to call css_get() first, because memcg_uncharge_kmem()
+ * Our caller must use css_get() first, because memcg_uncharge_kmem()
* will call css_put() if it sees the memcg is dead.
*/
smp_wmb();
_
But it's still not good.
- What is the smp_wmb() for? These barriers should always be
documented so readers can see what we're barriering against but this
one is floating around unexplained.
- What does memcg_uncharge_kmem() have to do with all this?
memcg_kmem_mark_dead() just does a set_bit() - what has that to do
with memcg_kmem_mark_dead().
So I dunno, it's all clear as mud and I hope we can do better.
> @@ -3060,8 +3065,16 @@ static void memcg_uncharge_kmem(struct mem_cgroup *memcg, u64 size)
> if (res_counter_uncharge(&memcg->kmem, size))
> return;
>
> + /*
> + * Releases a reference taken in kmem_cgroup_css_offline in case
> + * this last uncharge is racing with the offlining code or it is
> + * outliving the memcg existence.
> + *
> + * The memory barrier imposed by test&clear is paired with the
> + * explicit one in memcg_kmem_mark_dead().
> + */
OK, that clears things up a bit. A small bit.
This code is far too tricky :(
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