Re: [PATCH] mm/oom: Suppress unnecessary "sharing same memory" message.

[Michal Hocko <mhocko@suse.cz>]

On Mon 01-06-15 21:02:20, Tetsuo Handa wrote:
> Michal Hocko wrote:
> > On Sun 31-05-15 20:10:23, Tetsuo Handa wrote:
> > [...]
> > > By the way, I got two mumbles.
> > > 
> > > Is "If any of p's children has a different mm and is eligible for kill," logic
> > > in oom_kill_process() really needed? Didn't select_bad_process() which was
> > > called proior to calling oom_kill_process() already choose a best victim
> > > using for_each_process_thread() ?
> > 
> > This tries to have smaller effect on the system. It tries to kill
> > younger tasks because this might be and quite often is sufficient to
> > resolve the OOM condition.
> > 
> > > Is "/* mm cannot safely be dereferenced after task_unlock(victim) */" true?
> > > It seems to me that it should be "/* mm cannot safely be compared after
> > > task_unlock(victim) */" because it is theoretically possible to have
> > > 
> > >   CPU 0                         CPU 1                   CPU 2
> > >   task_unlock(victim);
> > >                                 victim exits and releases mm.
> > >                                 Usage count of the mm becomes 0 and thus released.
> > >                                                         New mm is allocated and assigned to some thread.
> > >   (p->mm == mm) matches the recreated mm and kill unrelated p.
> > > 
> > > sequence. We need to either get a reference to victim's mm before
> > > task_unlock(victim) or do comparison before task_unlock(victim).
> > 
> > Hmm, I guess you are right. The race is theoretically possible,
> > especially when there are many tasks when iterating over the list might
> > take some time. reference to the mm would solve this. Care to send a
> > patch?
> > 
> > -- 
> > Michal Hocko
> > SUSE Labs
> > 
> I see. Here is a patch.

It would be preferable to post this in a separate email thread.
Conflating different issues in the same thread has already proven messy.

> mmput() may sleep. But oom_kill_process() is a sleep-able context, right?

Sure.

> ----------------------------------------
> >From 15afd1f40b132719c323e81d58064ff7115206f9 Mon Sep 17 00:00:00 2001
> From: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
> Date: Mon, 1 Jun 2015 20:54:14 +0900
> Subject: [PATCH] mm,oom: Fix potentially killing unrelated process or
>  depleting memory.
> 
> At the for_each_process() loop in oom_kill_process(), we are comparing
> address of OOM victim's mm without holding a reference to that mm.
> If there are a lot of processes to compare or a lot of "Kill process
> %d (%s) sharing same memory" messages to print, for_each_process() loop
> could take very long time.
> 
> It is possible that meanwhile the OOM victim exits and releases its mm,
> and then mm is allocated with the same address and assigned to some
> unrelated process. When we hit such race, the unrelated process will be
> killed by error. To make sure that the OOM victim's mm does not go away
> until for_each_process() loop finishes, get a reference on the OOM
> victim's mm before calling task_unlock(victim).

OK

> Likewise, move do_send_sig_info(SIGKILL, victim) to before
> mark_oom_victim(victim) in case for_each_process() took very long time,
> for the OOM victim can abuse ALLOC_NO_WATERMARKS by TIF_MEMDIE via e.g.
> memset() in user space until SIGKILL is delivered.

This is unrelated and I believe even not necessary.

> Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
> ---
>  mm/oom_kill.c | 15 +++++++++------
>  1 file changed, 9 insertions(+), 6 deletions(-)
> 
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index dff991e..5eb1e65 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -559,14 +559,17 @@ void oom_kill_process(struct task_struct *p, gfp_t gfp_mask, int order,
>  		victim = p;
>  	}
>  
> -	/* mm cannot safely be dereferenced after task_unlock(victim) */
> +	/* Get a reference to safely compare mm after task_unlock(victim) */
>  	mm = victim->mm;
> -	mark_oom_victim(victim);
> +	atomic_inc(&mm->mm_users);
>  	pr_err("Killed process %d (%s) total-vm:%lukB, anon-rss:%lukB, file-rss:%lukB\n",
> -		task_pid_nr(victim), victim->comm, K(victim->mm->total_vm),
> -		K(get_mm_counter(victim->mm, MM_ANONPAGES)),
> -		K(get_mm_counter(victim->mm, MM_FILEPAGES)));
> +		task_pid_nr(victim), victim->comm, K(mm->total_vm),
> +		K(get_mm_counter(mm, MM_ANONPAGES)),
> +		K(get_mm_counter(mm, MM_FILEPAGES)));
>  	task_unlock(victim);
> +	/* Send SIGKILL before setting TIF_MEMDIE. */
> +	do_send_sig_info(SIGKILL, SEND_SIG_FORCED, victim, true);
> +	mark_oom_victim(victim);
>  
>  	/*
>  	 * Kill all user processes sharing victim->mm in other thread groups, if
> @@ -592,7 +595,7 @@ void oom_kill_process(struct task_struct *p, gfp_t gfp_mask, int order,
>  		}
>  	rcu_read_unlock();
>  
> -	do_send_sig_info(SIGKILL, SEND_SIG_FORCED, victim, true);
> +	mmput(mm);
>  	put_task_struct(victim);
>  }
>  #undef K
> -- 
> 1.8.3.1
> 
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-- 
Michal Hocko
SUSE Labs

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