On Thu 09-11-17 19:49:16, Tetsuo Handa wrote:
> Michal Hocko wrote:
> > On Wed 08-11-17 20:01:48, Tetsuo Handa wrote:
> > > Commit 212925802454672e ("mm: oom: let oom_reap_task and exit_mmap run
> > > concurrently") moved the location of setting MMF_OOM_SKIP from __mmput()
> > > in kernel/fork.c (which is used by both MMU and !MMU) to exit_mm() in
> > > mm/mmap.c (which is used by MMU only). As a result, that commit required
> > > OOM victims in !MMU kernels to disappear from the task list in order to
> > > reenable the OOM killer, for !MMU kernels can no longer set MMF_OOM_SKIP
> > > (unless the OOM victim's mm is shared with global init process).
> >
> > nack withtout demonstrating that the problem is real. It is true it
> > removes some lines but this is mostly this...
>
> Then, it is impossible unless somebody volunteers proving it.
> I'm not a nommu kernel user.
Do not convolute the code for a non-existent problem. Full stop.
[...]
> > On Wed 08-11-17 20:01:48, Tetsuo Handa wrote:
> > [...]
> > > @@ -829,7 +831,7 @@ static void oom_kill_process(struct oom_control *oc, const char *message)
> > > unsigned int victim_points = 0;
> > > static DEFINE_RATELIMIT_STATE(oom_rs, DEFAULT_RATELIMIT_INTERVAL,
> > > DEFAULT_RATELIMIT_BURST);
> > > - bool can_oom_reap = true;
> > > + bool can_oom_reap = IS_ENABLED(CONFIG_MMU);
> > >
> > > /*
> > > * If the task is already exiting, don't alarm the sysadmin or kill
> > > @@ -929,7 +931,6 @@ static void oom_kill_process(struct oom_control *oc, const char *message)
> > > continue;
> > > if (is_global_init(p)) {
> > > can_oom_reap = false;
> > > - set_bit(MMF_OOM_SKIP, &mm->flags);
> > > pr_info("oom killer %d (%s) has mm pinned by %d (%s)\n",
> > > task_pid_nr(victim), victim->comm,
> > > task_pid_nr(p), p->comm);
> > > @@ -947,6 +948,8 @@ static void oom_kill_process(struct oom_control *oc, const char *message)
> > >
> > > if (can_oom_reap)
> > > wake_oom_reaper(victim);
> > > + else
> > > + set_bit(MMF_OOM_SKIP, &mm->flags);
> > >
> > > mmdrop(mm);
> > > put_task_struct(victim);
> >
> > Also this looks completely broken. nommu kernels lose the premature oom
> > killing protection almost completely (they simply rely on the sleep
> > before dropping the oom_lock).
> >
>
> If you are worrying that setting MMF_OOM_SKIP immediately might cause
> premature OOM killing), what we would afford is timeout-based approach
> shown below, for it will be a waste of resource to add the OOM reaper kernel
> thread which does nothing but setting MMF_OOM_SKIP.
No! See above
--
Michal Hocko
SUSE Labs
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