mm/oom_kill.c: clean up oom_reap_task_mm()

Andrew has noticed some inconsistencies in oom_reap_task_mm. Notably

- Undocumented return value.

- comment "failed to reap part..." is misleading - sounds like it's
referring to something which happened in the past, is in fact
referring to something which might happen in the future.

- fails to call trace_finish_task_reaping() in one case

- code duplication.

- Increases mmap_sem hold time a little by moving
trace_finish_task_reaping() inside the locked region. So sue me ;)

- Sharing the finish: path means that the trace event won't
distinguish between the two sources of finishing.

Add a short explanation for the return value and fix the rest by
reorganizing the function a bit to have unified function exit paths.

Link: http://lkml.kernel.org/r/[e-mail]
Suggested-by: Andrew Morton <[e-mail]>
Signed-off-by: Michal Hocko <[e-mail]>
Reviewed-by: Andrew Morton <[e-mail]>
Cc: Tetsuo Handa <[e-mail]>
Cc: David Rientjes <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/oom_kill.c

···
 	return ret;
 }
 
+/*
+ * Reaps the address space of the give task.
+ *
+ * Returns true on success and false if none or part of the address space
+ * has been reclaimed and the caller should retry later.
+ */
 static bool oom_reap_task_mm(struct task_struct *tsk, struct mm_struct *mm)
 {
+	bool ret = true;
+
 	if (!down_read_trylock(&mm->mmap_sem)) {
 		trace_skip_task_reaping(tsk->pid);
 		return false;
···
 	 * down_write();up_write() cycle in exit_mmap().
 	 */
 	if (test_bit(MMF_OOM_SKIP, &mm->flags)) {
-		up_read(&mm->mmap_sem);
 		trace_skip_task_reaping(tsk->pid);
-		return true;
+		goto out_unlock;
 	}
 
 	trace_start_task_reaping(tsk->pid);
 
 	/* failed to reap part of the address space. Try again later */
-	if (!__oom_reap_task_mm(mm)) {
-		up_read(&mm->mmap_sem);
-		return false;
-	}
+	ret = __oom_reap_task_mm(mm);
+	if (!ret)
+		goto out_finish;
 
 	pr_info("oom_reaper: reaped process %d (%s), now anon-rss:%lukB, file-rss:%lukB, shmem-rss:%lukB\n",
 			task_pid_nr(tsk), tsk->comm,
 			K(get_mm_counter(mm, MM_ANONPAGES)),
 			K(get_mm_counter(mm, MM_FILEPAGES)),
 			K(get_mm_counter(mm, MM_SHMEMPAGES)));
+out_finish:
+	trace_finish_task_reaping(tsk->pid);
+out_unlock:
 	up_read(&mm->mmap_sem);
 
-	trace_finish_task_reaping(tsk->pid);
-	return true;
+	return ret;
 }
 
 #define MAX_OOM_REAP_RETRIES 10