[PATCHv2 4.3-rc6] proc: fix convert from oom_score_adj to oom_adj

["Hongjie Fang (方洪杰)" <Hongjie.Fang@spreadtrum.com>]

The oom_adj has been replaced by oom_score_adj in kernel,
but the /proc/pid/oom_adj is provided for legacy purposes.
When write/read a value into/from /proc/pid/oom_adj,
there is a transformation between oom_adj and oom_score_adj.

After writing a new value into /proc/pid/oom_adj, then read it.
The return value is a different value than you wrote.
Fix this by adding a adjustment factor.

Signed-off-by: Hongjie Fang <hongjie.fang@spreadtrum.com>
---
Encountered the problem when I changed a task's oom_adj on
an Android smart phone. As follows,
1. # cat /proc/1450/oom_adj
   15
2. # echo 10 > /proc/1450/oom_adj
3. # cat /proc/1450/oom_adj
   9


diff --git a/fs/proc/base.c b/fs/proc/base.c
index b25eee4..2312e43 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -1019,15 +1019,19 @@ static ssize_t oom_adj_read(struct file *file, char __user *buf, size_t count,
 	int oom_adj = OOM_ADJUST_MIN;
 	size_t len;
 	unsigned long flags;
+	int adjust;
 
 	if (!task)
 		return -ESRCH;
 	if (lock_task_sighand(task, &flags)) {
-		if (task->signal->oom_score_adj == OOM_SCORE_ADJ_MAX)
+		if (task->signal->oom_score_adj == OOM_SCORE_ADJ_MAX) {
 			oom_adj = OOM_ADJUST_MAX;
-		else
-			oom_adj = (task->signal->oom_score_adj * -OOM_DISABLE) /
+		} else {
+			adjust = task->signal->oom_score_adj > 0 ?
+				 (OOM_SCORE_ADJ_MAX-1) : -(OOM_SCORE_ADJ_MAX-1);
+			oom_adj = (task->signal->oom_score_adj * -OOM_DISABLE + adjust) /
 				  OOM_SCORE_ADJ_MAX;
+		}
 		unlock_task_sighand(task, &flags);
 	}
 	put_task_struct(task);