Re: [RFC PATCH v2 2/5] mm: Add policy_name to identify OOM policies

[Jonathan Corbet <corbet@lwn.net>]

Chuyi Zhou <zhouchuyi@bytedance.com> writes:

> This patch adds a new metadata policy_name in oom_control and report it
> in dump_header(), so we can know what has been the selection policy. In
> BPF program, we can call kfunc set_oom_policy_name to set the current
> user-defined policy name. The in-kernel policy_name is "default".
>
> Signed-off-by: Chuyi Zhou <zhouchuyi@bytedance.com>
> ---
>  include/linux/oom.h |  7 +++++++
>  mm/oom_kill.c       | 42 +++++++++++++++++++++++++++++++++++++++---
>  2 files changed, 46 insertions(+), 3 deletions(-)

So I have a possibly dumb question here...

> diff --git a/include/linux/oom.h b/include/linux/oom.h
> index 7d0c9c48a0c5..69d0f2ec6ea6 100644
> --- a/include/linux/oom.h
> +++ b/include/linux/oom.h
> @@ -22,6 +22,10 @@ enum oom_constraint {
>  	CONSTRAINT_MEMCG,
>  };
>  
> +enum {
> +	POLICY_NAME_LEN = 16,
> +};

We've defined our name length, fine...

>  /*
>   * Details of the page allocation that triggered the oom killer that are used to
>   * determine what should be killed.
> @@ -52,6 +56,9 @@ struct oom_control {
>  
>  	/* Used to print the constraint info. */
>  	enum oom_constraint constraint;
> +
> +	/* Used to report the policy info. */
> +	char policy_name[POLICY_NAME_LEN];
>  };

...that is the length of the array, appended to the structure...

>  extern struct mutex oom_lock;
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index 255c9ef1d808..3239dcdba4d7 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -443,6 +443,35 @@ static int dump_task(struct task_struct *p, void *arg)
>  	return 0;
>  }
>  
> +__bpf_kfunc void set_oom_policy_name(struct oom_control *oc, const char *src, size_t sz)
> +{
> +	memset(oc->policy_name, 0, sizeof(oc->policy_name));
> +
> +	if (sz > POLICY_NAME_LEN)
> +		sz = POLICY_NAME_LEN;
> +
> +	memcpy(oc->policy_name, src, sz);
> +}

This truncates the name, possibly leaving it without a terminating NUL
character, right?

> +__diag_push();
> +__diag_ignore_all("-Wmissing-prototypes",
> +		  "kfuncs which will be used in BPF programs");
> +
> +__weak noinline void bpf_set_policy_name(struct oom_control *oc)
> +{
> +}
> +
> +__diag_pop();
> +
> +BTF_SET8_START(bpf_oom_policy_kfunc_ids)
> +BTF_ID_FLAGS(func, set_oom_policy_name)
> +BTF_SET8_END(bpf_oom_policy_kfunc_ids)
> +
> +static const struct btf_kfunc_id_set bpf_oom_policy_kfunc_set = {
> +	.owner          = THIS_MODULE,
> +	.set            = &bpf_oom_policy_kfunc_ids,
> +};
> +
>  /**
>   * dump_tasks - dump current memory state of all system tasks
>   * @oc: pointer to struct oom_control
> @@ -484,8 +513,8 @@ static void dump_oom_summary(struct oom_control *oc, struct task_struct *victim)
>  
>  static void dump_header(struct oom_control *oc, struct task_struct *p)
>  {
> -	pr_warn("%s invoked oom-killer: gfp_mask=%#x(%pGg), order=%d, oom_score_adj=%hd\n",
> -		current->comm, oc->gfp_mask, &oc->gfp_mask, oc->order,
> +	pr_warn("%s invoked oom-killer: gfp_mask=%#x(%pGg), order=%d, policy_name=%s, oom_score_adj=%hd\n",
> +		current->comm, oc->gfp_mask, &oc->gfp_mask, oc->order, oc->policy_name,

...and if the policy name is unterminated, this print will run off the
end of the structure.

Am I missing something here?

Thanks,

jon