On Sat, 17 Jun 2017, Tetsuo Handa wrote:
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index 04c9143..cf1d331 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -470,38 +470,9 @@ static bool __oom_reap_task_mm(struct task_struct *tsk, struct mm_struct *mm)
> {
> struct mmu_gather tlb;
> struct vm_area_struct *vma;
> - bool ret = true;
> -
> - /*
> - * We have to make sure to not race with the victim exit path
> - * and cause premature new oom victim selection:
> - * __oom_reap_task_mm exit_mm
> - * mmget_not_zero
> - * mmput
> - * atomic_dec_and_test
> - * exit_oom_victim
> - * [...]
> - * out_of_memory
> - * select_bad_process
> - * # no TIF_MEMDIE task selects new victim
> - * unmap_page_range # frees some memory
> - */
> - mutex_lock(&oom_lock);
> -
> - if (!down_read_trylock(&mm->mmap_sem)) {
> - ret = false;
> - goto unlock_oom;
> - }
>
> - /*
> - * increase mm_users only after we know we will reap something so
> - * that the mmput_async is called only when we have reaped something
> - * and delayed __mmput doesn't matter that much
> - */
> - if (!mmget_not_zero(mm)) {
> - up_read(&mm->mmap_sem);
> - goto unlock_oom;
> - }
> + if (!down_read_trylock(&mm->mmap_sem))
> + return false;
>
> /*
> * Tell all users of get_user/copy_from_user etc... that the content
> @@ -537,16 +508,7 @@ static bool __oom_reap_task_mm(struct task_struct *tsk, struct mm_struct *mm)
> K(get_mm_counter(mm, MM_FILEPAGES)),
> K(get_mm_counter(mm, MM_SHMEMPAGES)));
> up_read(&mm->mmap_sem);
> -
> - /*
> - * Drop our reference but make sure the mmput slow path is called from a
> - * different context because we shouldn't risk we get stuck there and
> - * put the oom_reaper out of the way.
> - */
> - mmput_async(mm);
> -unlock_oom:
> - mutex_unlock(&oom_lock);
> - return ret;
> + return true;
> }
>
> #define MAX_OOM_REAP_RETRIES 10
> @@ -569,12 +531,31 @@ static void oom_reap_task(struct task_struct *tsk)
>
> done:
> tsk->oom_reaper_list = NULL;
> + /*
> + * Drop a mm_users reference taken by mark_oom_victim().
> + * A mm_count reference taken by mark_oom_victim() remains.
> + */
> + mmput_async(mm);
This doesn't prevent serial oom killing for either the system oom killer
or for the memcg oom killer.
The oom killer cannot detect tsk_is_oom_victim() if the task has either
been removed from the tasklist or has already done cgroup_exit(). For
memcg oom killings in particular, cgroup_exit() is usually called very
shortly after the oom killer has sent the SIGKILL. If the oom reaper does
not fail (for example by failing to grab mm->mmap_sem) before another
memcg charge after cgroup_exit(victim), additional processes are killed
because the iteration does not view the victim.
This easily kills all processes attached to the memcg with no memory
freeing from any victim.
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