Re: [patch] mm, oom: prevent additional oom kills before memory is freed

[Michal Hocko <mhocko@kernel.org>]

On Fri 16-06-17 21:22:20, Tetsuo Handa wrote:
> Michal Hocko wrote:
> > OK, could you play with the patch/idea suggested in
> > http://lkml.kernel.org/r/20170615122031.GL1486@dhcp22.suse.cz?
> 
> I think we don't need to worry about mmap_sem dependency inside __mmput().
> Since the OOM killer checks for !MMF_OOM_SKIP mm rather than TIF_MEMDIE thread,
> we can keep the OOM killer disabled until we set MMF_OOM_SKIP to the victim's mm.
> That is, elevating mm_users throughout the reaping procedure does not cause
> premature victim selection, even after TIF_MEMDIE is cleared from the victim's
> thread. Then, we don't need to use down_write()/up_write() for non OOM victim's mm
> (nearly 100% of exit_mmap() calls), and can force partial reaping of OOM victim's mm
> (nearly 0% of exit_mmap() calls) before __mmput() starts doing exit_aio() etc.
> Patch is shown below. Only compile tested.

Yes, that would be another approach.
 
>  include/linux/sched/coredump.h |  1 +
>  mm/oom_kill.c                  | 80 ++++++++++++++++++++----------------------
>  2 files changed, 40 insertions(+), 41 deletions(-)
> 
> diff --git a/include/linux/sched/coredump.h b/include/linux/sched/coredump.h
> index 98ae0d0..6b6237b 100644
> --- a/include/linux/sched/coredump.h
> +++ b/include/linux/sched/coredump.h
> @@ -62,6 +62,7 @@ static inline int get_dumpable(struct mm_struct *mm)
>   * on NFS restore
>   */
>  //#define MMF_EXE_FILE_CHANGED	18	/* see prctl_set_mm_exe_file() */
> +#define MMF_OOM_REAPING		18	/* mm is supposed to be reaped */

A new flag is not really needed. We can increase it for _each_ reapable
oom victim.

> @@ -658,6 +643,13 @@ static void mark_oom_victim(struct task_struct *tsk)
>  	if (!cmpxchg(&tsk->signal->oom_mm, NULL, mm))
>  		mmgrab(tsk->signal->oom_mm);
>  
> +#ifdef CONFIG_MMU
> +	if (!test_bit(MMF_OOM_REAPING, &mm->flags)) {
> +		set_bit(MMF_OOM_REAPING, &mm->flags);
> +		mmget(mm);
> +	}
> +#endif

This would really need a big fat warning explaining why we do not need
mmget_not_zero. We rely on exit_mm doing both mmput and tsk->mm = NULL
under the task_lock and mark_oom_victim is called under this lock as
well and task_will_free_mem resp. find_lock_task_mm makes sure we do not
even consider tasks wihout mm.

I agree that a solution which is fully contained inside the oom proper
would be preferable to touching __mmput path.
-- 
Michal Hocko
SUSE Labs

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