Re: [PATCH v2] mm, memcg: fix inconsistent oom event behavior

[Johannes Weiner <hannes@cmpxchg.org>]

On Wed, Apr 22, 2020 at 07:06:43AM -0400, Yafang Shao wrote:
> A recent commit 9852ae3fe529 ("mm, memcg: consider subtrees in
> memory.events") changes the behavior of memcg events, which will
> consider subtrees in memory.events. But oom_kill event is a special one
> as it is used in both cgroup1 and cgroup2. In cgroup1, it is displayed
> in memory.oom_control. The file memory.oom_control is in both root memcg
> and non root memcg, that is different with memory.event as it only in
> non-root memcg. That commit is okay for cgroup2, but it is not okay for
> cgroup1 as it will cause inconsistent behavior between root memcg and
> non-root memcg.
> 
> Here's an example on why this behavior is inconsistent in cgroup1.
>      root memcg
>      /
>   memcg foo
>    /
> memcg bar
> 
> Suppose there's an oom_kill in memcg bar, then the oon_kill will be
> 
>      root memcg : memory.oom_control(oom_kill)  0
>      /
>   memcg foo : memory.oom_control(oom_kill)  1
>    /
> memcg bar : memory.oom_control(oom_kill)  1
> 
> For the non-root memcg, its memory.oom_control(oom_kill) includes its
> descendants' oom_kill, but for root memcg, it doesn't include its
> descendants' oom_kill. That means, memory.oom_control(oom_kill) has
> different meanings in different memcgs. That is inconsistent. Then the user
> has to know whether the memcg is root or not.
> 
> If we can't fully support it in cgroup1, for example by adding
> memory.events.local into cgroup1 as well, then let's don't touch
> its original behavior.
> 
> Setting CGRP_ROOT_MEMORY_LOCAL_EVENTS for legacy hierarchy by
> default rather than special casing it somewhere quite deep in the code
> would be better, per discussion with Michal.
> 
> Fixes: 9852ae3fe529 ("mm, memcg: consider subtrees in memory.events")
> Cc: Chris Down <chris@chrisdown.name>
> Cc: Shakeel Butt <shakeelb@google.com>
> Cc: Michal Hocko <mhocko@kernel.org>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
> ---
>  mm/memcontrol.c | 14 ++++++++++++--
>  1 file changed, 12 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 5beea03dd58a..0f7381bddcee 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -5940,10 +5940,20 @@ static void mem_cgroup_bind(struct cgroup_subsys_state *root_css)
>  	 * guarantees that @root doesn't have any children, so turning it
>  	 * on for the root memcg is enough.
>  	 */
> -	if (cgroup_subsys_on_dfl(memory_cgrp_subsys))
> +	if (cgroup_subsys_on_dfl(memory_cgrp_subsys)) {
>  		root_mem_cgroup->use_hierarchy = true;
> -	else
> +	} else {
>  		root_mem_cgroup->use_hierarchy = false;
> +		/*
> +		 * Set CGRP_ROOT_MEMORY_LOCAL_EVENTS for legacy hierarchy
> +		 * by default to avoid inconsistent oom_kill behavior
> +		 * between root memcg and non-root memcg.
> +		 * Regarding default hierarchy, as this flag will be set
> +		 * or cleared later, we don't need to process it in this
> +		 * function.
> +		 */
> +		cgrp_dfl_root.flags |= CGRP_ROOT_MEMORY_LOCAL_EVENTS;

That will cause problems for people trying to remount. From
cgroup1_reconfigure():

	/* Don't allow flags or name to change at remount */
	if ((ctx->flags ^ root->flags) ||
	    (ctx->name && strcmp(ctx->name, root->name))) {
		errorfc(fc, "option or name mismatch, new: 0x%x \"%s\", old: 0x%x \"%s\"",
		       ctx->flags, ctx->name ?: "", root->flags, root->name);
		ret = -EINVAL;
		goto out_unlock;
	}

These flags belong to the user, they're read-only to the cgroup
implementation. Let's not mess with them from a controller.

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 4b868e5a687f..e831a90b5506 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -773,6 +773,8 @@ static inline void memcg_memory_event(struct mem_cgroup *memcg,
 		atomic_long_inc(&memcg->memory_events[event]);
 		cgroup_file_notify(&memcg->events_file);
 
+		if (!cgroup_subsys_on_dfl(memory_cgrp_subsys))
+			break;
 		if (cgrp_dfl_root.flags & CGRP_ROOT_MEMORY_LOCAL_EVENTS)
 			break;
 	} while ((memcg = parent_mem_cgroup(memcg)) &&