mm, oom: tighten task_will_free_mem() locking

"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.

Link: http://lkml.kernel.org/r/1467201562-6709-1-git-send-email-mhocko@kernel.org
Signed-off-by: Michal Hocko <mhocko@suse.com>
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Vladimir Davydov <vdavydov@parallels.com>
Cc: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 4c21f74..7d0a275 100644 (file)
--- 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;