Re: [patch -mm] oom: avoid deferring oom killer if exiting task is being traced

[Oleg Nesterov <oleg@redhat.com>]

On 03/12, David Rientjes wrote:
>
> The oom killer naturally defers killing anything if it finds an eligible
> task that is already exiting and has yet to detach its ->mm.  This avoids
> unnecessarily killing tasks when one is already in the exit path and may
> free enough memory that the oom killer is no longer needed.  This is
> detected by PF_EXITING since threads that have already detached its ->mm
> are no longer considered at all.

So, this is on top of oom-prevent-unnecessary-oom-kills-or-kernel-panics.patch

I still can't understand which problems that patch tries to solve. Could
you explain why this patch helps in details?

Speaking of "already exiting and has yet to detach its ->mm", did you look
at "[PATCH 2/3] oom: select_bad_process: improve the PF_EXITING check" ?

> The problem with always deferring when a thread is PF_EXITING, however,
> is that it may never actually exit when being traced, specifically if
> another task is tracing it with PTRACE_O_TRACEEXIT.  The oom killer does
> not want to defer in this case since there is no guarantee that thread
> will ever exit without intervention.

IOW, you are trying to fight with the test-case I sent,

> +			} else {
> +				/*
> +				 * If this task is not being ptraced on exit,
> +				 * then wait for it to finish before killing
> +				 * some other task unnecessarily.
> +				 */
> +				if (!(task_ptrace(p->group_leader) &
> +							PT_TRACE_EXIT))
> +					return ERR_PTR(-1UL);

No, this can't help afaics. It is trivial to change the exploit and
get the same result.

Perhaps I missed something, I didn't read this patch carefully. Will
try to do later.

However. could you please answer my question above?


Also. We have the serious and easily exploitable bugs, I think we
should fix them first. I'll report more details later today.

Oleg.

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