linux-kernel - Re: [PATCH] mm: oom: Fix race condition between oom_badness and do

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Date:   Fri, 9 Mar 2018 19:48:46 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     gkohli@...eaurora.org, rientjes@...gle.com
Cc:     akpm@...ux-foundation.org, mhocko@...e.com,
        kirill.shutemov@...ux.intel.com, aarcange@...hat.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linux-arm-msm@...r.kernel.org
Subject: Re: [PATCH] mm: oom: Fix race condition between oom_badness and do_exit of task

Kohli, Gaurav wrote:
> > t->alloc_lock is still held when leaving find_lock_task_mm(), which means
> > that t->mm != NULL. But nothing prevents t from setting t->mm = NULL at
> > exit_mm() from do_exit() and calling exit_creds() from __put_task_struct(t)
> > after task_unlock(t) is called. Seems difficult to trigger race window. Maybe
> > something has preempted because oom_badness() becomes outside of RCU grace
> > period upon leaving find_lock_task_mm() when called from proc_oom_score().
> 
> Hi Tetsuo,
> 
> Yes it is not easy to reproduce seen twice till now and i agree with
> your analysis. But David has already fixing this in different way,
> So that also looks better to me:
> 
> https://patchwork.kernel.org/patch/10265641/
> 

Yes, I'm aware of that patch.

> But if need to keep that code, So we have to bump up the task
> reference that's only i can think of now.

I don't think so, for I think it is safe to call
has_capability_noaudit(p) with p->alloc_lock held.

diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index f2e7dfb..4efcfb8 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -222,7 +222,6 @@ unsigned long oom_badness(struct task_struct *p, struct mem_cgroup *memcg,
 	 */
 	points = get_mm_rss(p->mm) + get_mm_counter(p->mm, MM_SWAPENTS) +
 		mm_pgtables_bytes(p->mm) / PAGE_SIZE;
-	task_unlock(p);
 
 	/*
 	 * Root processes get 3% bonus, just like the __vm_enough_memory()
@@ -230,6 +229,7 @@ unsigned long oom_badness(struct task_struct *p, struct mem_cgroup *memcg,
 	 */
 	if (has_capability_noaudit(p, CAP_SYS_ADMIN))
 		points -= (points * 3) / 100;
+	task_unlock(p);
 
 	/* Normalize to oom_score_adj units */
 	adj *= totalpages / 1000;