linux-kernel - [PATCH -mm] mm/oom_kill: change oom_kill_process() to bump ->mm_count rather than ->mm

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Message-ID: <20151005163427.GA20595@redhat.com>
Date:	Mon, 5 Oct 2015 18:34:27 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	David Rientjes <rientjes@...gle.com>,
	Kyle Walker <kwalker@...hat.com>,
	Michal Hocko <mhocko@...nel.org>,
	Stanislav Kozina <skozina@...hat.com>,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	linux-kernel@...r.kernel.org
Subject: [PATCH -mm] mm/oom_kill: change oom_kill_process() to bump
	->mm_count rather than ->mm_users

oom_kill_process() does atomic_inc(&mm->mm_users) to ensure that
this ->mm can't go away and this is wrong, change it to rely on
->mm_count and mmdrop().

Firstly, we do not want to delay exit_mmap/etc if the victim exits
before we do mmput(), but this is minor.

More importantly, we simply can not do mmput() in oom_kill_process(),
this can deadlock. For example, suppose that access_process_vm(tsk)
triggers OOM and oom-killer decides to kill this "tsk". If it exits
and does mmput() before us, ksm_exit() called by us may want to want
to take the same mmap_sem for writing.

Signed-off-by: Oleg Nesterov <oleg@...hat.com>
---
 mm/oom_kill.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 034d219..52abb78 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -571,7 +571,7 @@ void oom_kill_process(struct oom_control *oc, struct task_struct *p,
 
 	/* Get a reference to safely compare mm after task_unlock(victim) */
 	mm = victim->mm;
-	atomic_inc(&mm->mm_users);
+	atomic_inc(&mm->mm_count);
 	/*
 	 * We should send SIGKILL before setting TIF_MEMDIE in order to prevent
 	 * the OOM victim from depleting the memory reserves from the user
@@ -609,7 +609,7 @@ void oom_kill_process(struct oom_control *oc, struct task_struct *p,
 	}
 	rcu_read_unlock();
 
-	mmput(mm);
+	mmdrop(mm);
 	put_task_struct(victim);
 }
 #undef K
-- 
2.4.3


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