Re: [PATCH] mm/oom_kill: set oc->constraint in constrained_alloc()

[Yafang Shao <laoar.shao@gmail.com>]

On Fri, Jun 14, 2019 at 2:56 AM Michal Hocko <mhocko@suse.com> wrote:
>
> On Thu 13-06-19 21:55:50, Yafang Shao wrote:
> > In dump_oom_summary() oc->constraint is used to show
> > oom_constraint_text, but it hasn't been set before.
> > So the value of it is always the default value 0.
> > We should set it in constrained_alloc().
>
> Thanks for catching that.
>
> > Bellow is the output when memcg oom occurs,
> >
> > before this patch:
> > [  133.078102] oom-kill:constraint=CONSTRAINT_NONE,nodemask=(null),
> > cpuset=/,mems_allowed=0,oom_memcg=/foo,task_memcg=/foo,task=bash,pid=7997,uid=0
> >
> > after this patch:
> > [  952.977946] oom-kill:constraint=CONSTRAINT_MEMCG,nodemask=(null),
> > cpuset=/,mems_allowed=0,oom_memcg=/foo,task_memcg=/foo,task=bash,pid=13681,uid=0
> >
>
> unless I am missing something
> Fixes: ef8444ea01d7 ("mm, oom: reorganize the oom report in dump_header")
>
> The patch looks correct but I think it is more complicated than it needs
> to be. Can we do the following instead?
>
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index 5a58778c91d4..f719b64741d6 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -987,8 +987,7 @@ static void oom_kill_process(struct oom_control *oc, const char *message)
>  /*
>   * Determines whether the kernel must panic because of the panic_on_oom sysctl.
>   */
> -static void check_panic_on_oom(struct oom_control *oc,
> -                              enum oom_constraint constraint)
> +static void check_panic_on_oom(struct oom_control *oc)
>  {
>         if (likely(!sysctl_panic_on_oom))
>                 return;
> @@ -998,7 +997,7 @@ static void check_panic_on_oom(struct oom_control *oc,
>                  * does not panic for cpuset, mempolicy, or memcg allocation
>                  * failures.
>                  */
> -               if (constraint != CONSTRAINT_NONE)
> +               if (oc->constraint != CONSTRAINT_NONE)
>                         return;
>         }
>         /* Do not panic for oom kills triggered by sysrq */
> @@ -1035,7 +1034,6 @@ EXPORT_SYMBOL_GPL(unregister_oom_notifier);
>  bool out_of_memory(struct oom_control *oc)
>  {
>         unsigned long freed = 0;
> -       enum oom_constraint constraint = CONSTRAINT_NONE;
>
>         if (oom_killer_disabled)
>                 return false;
> @@ -1071,10 +1069,10 @@ bool out_of_memory(struct oom_control *oc)
>          * Check if there were limitations on the allocation (only relevant for
>          * NUMA and memcg) that may require different handling.
>          */
> -       constraint = constrained_alloc(oc);
> -       if (constraint != CONSTRAINT_MEMORY_POLICY)
> +       oc->constraint = constrained_alloc(oc);
> +       if (oc->constraint != CONSTRAINT_MEMORY_POLICY)
>                 oc->nodemask = NULL;
> -       check_panic_on_oom(oc, constraint);
> +       check_panic_on_oom(oc);
>
>         if (!is_memcg_oom(oc) && sysctl_oom_kill_allocating_task &&
>             current->mm && !oom_unkillable_task(current, NULL, oc->nodemask) &&
>
> I guess the current confusion comes from the fact that we have
> constraint both in the oom_control and a local variable so I would
> rather remove that. What do you think?

Remove the local variable is fine by me.

Thanks
Yafang