Re: [PATCH] memcg: oom: ignore oom warnings from memory.max

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

On Fri, May 1, 2020 at 2:27 AM Shakeel Butt <shakeelb@google.com> wrote:
>
> Lowering memory.max can trigger an oom-kill if the reclaim does not
> succeed. However if oom-killer does not find a process for killing, it
> dumps a lot of warnings.
>

I have been confused by this behavior for several months and I think
it will confuse more memcg users.
We should keep the memcg oom behavior consistent with system oom - no
oom kill if no process.

What about bellow change ?

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index e28098e13f1c..25fbc37a747f 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -6086,6 +6086,9 @@ static ssize_t memory_max_write(struct
kernfs_open_file *of,
                        continue;
                }

+               if (!cgroup_is_populated(memcg->css.cgroup))
+                       break;
+
                memcg_memory_event(memcg, MEMCG_OOM);
                if (!mem_cgroup_out_of_memory(memcg, GFP_KERNEL, 0))
                        break;

> Deleting a memcg does not reclaim memory from it and the memory can
> linger till there is a memory pressure. One normal way to proactively
> reclaim such memory is to set memory.max to 0 just before deleting the
> memcg. However if some of the memcg's memory is pinned by others, this
> operation can trigger an oom-kill without any process and thus can log a
> lot un-needed warnings. So, ignore all such warnings from memory.max.
>
> Signed-off-by: Shakeel Butt <shakeelb@google.com>
> ---
>  include/linux/oom.h | 3 +++
>  mm/memcontrol.c     | 9 +++++----
>  mm/oom_kill.c       | 2 +-
>  3 files changed, 9 insertions(+), 5 deletions(-)
>
> diff --git a/include/linux/oom.h b/include/linux/oom.h
> index c696c265f019..6345dc55df64 100644
> --- a/include/linux/oom.h
> +++ b/include/linux/oom.h
> @@ -52,6 +52,9 @@ struct oom_control {
>
>         /* Used to print the constraint info. */
>         enum oom_constraint constraint;
> +
> +       /* Do not warn even if there is no process to be killed. */
> +       bool no_warn;
>  };
>
>  extern struct mutex oom_lock;
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 317dbbaac603..a1f00d9b9bb0 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1571,7 +1571,7 @@ unsigned long mem_cgroup_size(struct mem_cgroup *memcg)
>  }
>
>  static bool mem_cgroup_out_of_memory(struct mem_cgroup *memcg, gfp_t gfp_mask,
> -                                    int order)
> +                                    int order, bool no_warn)
>  {
>         struct oom_control oc = {
>                 .zonelist = NULL,
> @@ -1579,6 +1579,7 @@ static bool mem_cgroup_out_of_memory(struct mem_cgroup *memcg, gfp_t gfp_mask,
>                 .memcg = memcg,
>                 .gfp_mask = gfp_mask,
>                 .order = order,
> +               .no_warn = no_warn,
>         };
>         bool ret;
>
> @@ -1821,7 +1822,7 @@ static enum oom_status mem_cgroup_oom(struct mem_cgroup *memcg, gfp_t mask, int
>                 mem_cgroup_oom_notify(memcg);
>
>         mem_cgroup_unmark_under_oom(memcg);
> -       if (mem_cgroup_out_of_memory(memcg, mask, order))
> +       if (mem_cgroup_out_of_memory(memcg, mask, order, false))
>                 ret = OOM_SUCCESS;
>         else
>                 ret = OOM_FAILED;
> @@ -1880,7 +1881,7 @@ bool mem_cgroup_oom_synchronize(bool handle)
>                 mem_cgroup_unmark_under_oom(memcg);
>                 finish_wait(&memcg_oom_waitq, &owait.wait);
>                 mem_cgroup_out_of_memory(memcg, current->memcg_oom_gfp_mask,
> -                                        current->memcg_oom_order);
> +                                        current->memcg_oom_order, false);
>         } else {
>                 schedule();
>                 mem_cgroup_unmark_under_oom(memcg);
> @@ -6106,7 +6107,7 @@ static ssize_t memory_max_write(struct kernfs_open_file *of,
>                 }
>
>                 memcg_memory_event(memcg, MEMCG_OOM);
> -               if (!mem_cgroup_out_of_memory(memcg, GFP_KERNEL, 0))
> +               if (!mem_cgroup_out_of_memory(memcg, GFP_KERNEL, 0, true))
>                         break;
>         }
>
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index 463b3d74a64a..5ace39f6fe1e 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -1098,7 +1098,7 @@ bool out_of_memory(struct oom_control *oc)
>
>         select_bad_process(oc);
>         /* Found nothing?!?! */
> -       if (!oc->chosen) {
> +       if (!oc->chosen && !oc->no_warn) {
>                 dump_header(oc, NULL);
>                 pr_warn("Out of memory and no killable processes...\n");
>                 /*
> --
> 2.26.2.526.g744177e7f7-goog
>
>


-- 
Thanks
Yafang