[PATCH] mmotm: mm-oom-fortify-task_will_free_mem-fix

Michal Hocko
2016-06-29T11:59:33Z

From: Michal Hocko <mhocko@suse.com>

"mm, oom: fortify task_will_free_mem" has dropped task_lock around
task_will_free_mem in oom_kill_process bacause it assumed that a
potential race when the selected task exits will not be a problem
as the oom_reaper will call exit_oom_victim.

Tetsuo was objecting that nommu doesn't have oom_reaper so the race
would be still possible. The code would be racy and lockup prone
theoretically in other aspects without the oom reaper anyway so I didn't
considered this a big deal. But it seems that further changes I am
planning in this area will benefit from stable task->mm in this path as
well. So let's drop find_lock_task_mm from task_will_free_mem and call
it from under task_lock as we did previously. Just pull the task->mm !=
NULL check inside the function.

Andrew, could you please fold this into
mm-oom-fortify-task_will_free_mem-fix.patch?

Signed-off-by: Michal Hocko <mhocko@suse.com>
---
mm/oom_kill.c | 41 +++++++++++++++--------------------------
1 file changed, 15 insertions(+), 26 deletions(-)

diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 4c21f744daa6..7d0a275df822 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -757,45 +757,35 @@ static inline bool __task_will_free_mem(struct task_struct *task)
* Checks whether the given task is dying or exiting and likely to
* release its address space. This means that all threads and processes
* sharing the same mm have to be killed or exiting.
+ * Caller has to make sure that task->mm is stable (hold task_lock or
+ * it operates on the current).
*/
bool task_will_free_mem(struct task_struct *task)
{
- struct mm_struct *mm;
+ struct mm_struct *mm = task->mm;
struct task_struct *p;
bool ret;

- if (!__task_will_free_mem(task))
- return false;
-
/*
- * If the process has passed exit_mm we have to skip it because
- * we have lost a link to other tasks sharing this mm, we do not
- * have anything to reap and the task might then get stuck waiting
- * for parent as zombie and we do not want it to hold TIF_MEMDIE
+ * Skip tasks without mm because it might have passed its exit_mm and
+ * exit_oom_victim. oom_reaper could have rescued that but do not rely
+ * on that for now. We can consider find_lock_task_mm in future.
*/
- p = find_lock_task_mm(task);
- if (!p)
+ if (!mm)
return false;

- mm = p->mm;
+ if (!__task_will_free_mem(task))
+ return false;

/*
* This task has already been drained by the oom reaper so there are
* only small chances it will free some more
*/
- if (test_bit(MMF_OOM_REAPED, &mm->flags)) {
- task_unlock(p);
+ if (test_bit(MMF_OOM_REAPED, &mm->flags))
return false;
- }

- if (atomic_read(&mm->mm_users) <= 1) {
- task_unlock(p);
+ if (atomic_read(&mm->mm_users) <= 1)
return true;
- }
-
- /* pin the mm to not get freed and reused */
- atomic_inc(&mm->mm_count);
- task_unlock(p);

/*
* This is really pessimistic but we do not have any reliable way
@@ -812,7 +802,6 @@ bool task_will_free_mem(struct task_struct *task)
break;
}
rcu_read_unlock();
- mmdrop(mm);

return ret;
}
@@ -838,12 +827,15 @@ void oom_kill_process(struct oom_control *oc, struct task_struct *p,
* If the task is already exiting, don't alarm the sysadmin or kill
* its children or threads, just set TIF_MEMDIE so it can die quickly
*/
+ task_lock(p);
if (task_will_free_mem(p)) {
mark_oom_victim(p);
wake_oom_reaper(p);
+ task_unlock(p);
put_task_struct(p);
return;
}
+ task_unlock(p);

if (__ratelimit(&oom_rs))
dump_header(oc, p);
@@ -1014,11 +1006,8 @@ bool out_of_memory(struct oom_control *oc)
* If current has a pending SIGKILL or is exiting, then automatically
* select it. The goal is to allow it to allocate so that it may
* quickly exit and free its memory.
- *
- * But don't select if current has already released its mm and cleared
- * TIF_MEMDIE flag at exit_mm(), otherwise an OOM livelock may occur.
*/
- if (current->mm && task_will_free_mem(current)) {
+ if (task_will_free_mem(current)) {
mark_oom_victim(current);
wake_oom_reaper(current);
return true;
--
2.8.1