[RFC PATCH 4/6] oom, oom_reaper: consider mmget_not_zero as a failure

[Michal Hocko <mhocko@kernel.org>]

From: Michal Hocko <mhocko@suse.com>

mmget_not_zero failing means that we are racing with mmput->__mmput
and we are currently interpreting this as a success because we believe
that __mmput will release the address space. This is not guaranteed
though because at least exit_aio might wait on IO and it is not entirely
clear whether it will terminate in a bounded amount of time. It is hard
to tell what else is lurking there.

This patch makes this path more conservative and we report a failure
which will lead to setting MMF_OOM_NOT_REAPABLE and MMF_OOM_REAPED if
this state is permanent.

Signed-off-by: Michal Hocko <mhocko@suse.com>
---
 mm/oom_kill.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 4ac089cba353..b2210b6c38ba 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -460,7 +460,7 @@ static bool __oom_reap_task(struct task_struct *tsk)
 	struct mm_struct *mm = NULL;
 	struct zap_details details = {.check_swap_entries = true,
 				      .ignore_dirty = true};
-	bool ret = true;
+	bool ret = false;
 
 	/*
 	 * We have to make sure to not race with the victim exit path
@@ -479,10 +479,8 @@ static bool __oom_reap_task(struct task_struct *tsk)
 	mutex_lock(&oom_lock);
 
 	mm = tsk->signal->oom_mm;
-	if (!down_read_trylock(&mm->mmap_sem)) {
-		ret = false;
+	if (!down_read_trylock(&mm->mmap_sem))
 		goto unlock_oom;
-	}
 
 	/*
 	 * increase mm_users only after we know we will reap something so
@@ -494,6 +492,7 @@ static bool __oom_reap_task(struct task_struct *tsk)
 		goto unlock_oom;
 	}
 
+	ret = true;
 	tlb_gather_mmu(&tlb, mm, 0, -1);
 	for (vma = mm->mmap ; vma; vma = vma->vm_next) {
 		if (is_vm_hugetlb_page(vma))
-- 
2.8.1

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