On Wed 14-06-17 16:43:03, David Rientjes wrote:
> If mm->mm_users is not incremented because it is already zero by the oom
> reaper, meaning the final refcount has been dropped, do not set
> MMF_OOM_SKIP prematurely.
>
> __mmput() may not have had a chance to do exit_mmap() yet, so memory from
> a previous oom victim is still mapped.
true and do we have a _guarantee_ it will do it? E.g. can somebody block
exit_aio from completing? Or can somebody hold mmap_sem and thus block
ksm_exit resp. khugepaged_exit from completing? The reason why I was
conservative and set such a mm as MMF_OOM_SKIP was because I couldn't
give a definitive answer to those questions. And we really _want_ to
have a guarantee of a forward progress here. Killing an additional
proecess is a price to pay and if that doesn't trigger normall it sounds
like a reasonable compromise to me.
> __mput() naturally requires no
> references on mm->mm_users to do exit_mmap().
>
> Without this, several processes can be oom killed unnecessarily and the
> oom log can show an abundance of memory available if exit_mmap() is in
> progress at the time the process is skipped.
Have you seen this happening in the real life?
> Signed-off-by: David Rientjes <rientjes@xxxxxxxxxx>
> ---
> mm/oom_kill.c | 13 ++++++-------
> 1 file changed, 6 insertions(+), 7 deletions(-)
>
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -531,6 +531,7 @@ static bool __oom_reap_task_mm(struct task_struct *tsk, struct mm_struct *mm)
> NULL);
> }
> tlb_finish_mmu(&tlb, 0, -1);
> + set_bit(MMF_OOM_SKIP, &mm->flags);
> pr_info("oom_reaper: reaped process %d (%s), now anon-rss:%lukB, file-rss:%lukB, shmem-rss:%lukB\n",
> task_pid_nr(tsk), tsk->comm,
> K(get_mm_counter(mm, MM_ANONPAGES)),
> @@ -562,7 +563,11 @@ static void oom_reap_task(struct task_struct *tsk)
> if (attempts <= MAX_OOM_REAP_RETRIES)
> goto done;
>
> -
> + /*
> + * Hide this mm from OOM killer because it cannot be reaped since
> + * mm->mmap_sem cannot be acquired.
> + */
> + set_bit(MMF_OOM_SKIP, &mm->flags);
> pr_info("oom_reaper: unable to reap pid:%d (%s)\n",
> task_pid_nr(tsk), tsk->comm);
> debug_show_all_locks();
> @@ -570,12 +575,6 @@ static void oom_reap_task(struct task_struct *tsk)
> done:
> tsk->oom_reaper_list = NULL;
>
> - /*
> - * Hide this mm from OOM killer because it has been either reaped or
> - * somebody can't call up_write(mmap_sem).
> - */
> - set_bit(MMF_OOM_SKIP, &mm->flags);
> -
> /* Drop a reference taken by wake_oom_reaper */
> put_task_struct(tsk);
> }
--
Michal Hocko
SUSE Labs
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