[PATCH] mm: Warn mmput() from memory reclaim context.

[Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>]

syzbot is reporting that mmput() from shrinker function has a risk of
deadlock [1], for delayed_uprobe_add() from update_ref_ctr() calls
kzalloc(GFP_KERNEL) with delayed_uprobe_lock held, and
uprobe_clear_state() from __mmput() also holds delayed_uprobe_lock.

However, it took 18 months to hit this race for the third time, for
mmput() invokes __mmput() only when ->mm_users dropped to 0. If we
always warn like might_sleep(), we can detect the possibility of
deadlock more easier.

For now, I inlined the check under CONFIG_PROVE_LOCKING. If we find
more locations, we could introduce a macro like might_sleep().

[1] https://syzkaller.appspot.com/bug?id=bc9e7303f537c41b2b0cc2dfcea3fc42964c2d45

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
---
 kernel/fork.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/fork.c b/kernel/fork.c
index efc5493203ae..8717ce50ff0d 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1109,6 +1109,10 @@ static inline void __mmput(struct mm_struct *mm)
 void mmput(struct mm_struct *mm)
 {
 	might_sleep();
+#ifdef CONFIG_PROVE_LOCKING
+	/* Calling mmput() from shrinker context can deadlock. */
+	WARN_ON(current->flags & PF_MEMALLOC);
+#endif
 
 	if (atomic_dec_and_test(&mm->mm_users))
 		__mmput(mm);
-- 
2.18.4