oom: allow a non-CAP_SYS_RESOURCE proces to oom_score_adj down

We'd like to be able to oom_score_adj a process up/down as it
enters/leaves the foreground.  Currently, it is not possible to oom_adj
down without CAP_SYS_RESOURCE.  This patch allows a task to decrease its
oom_score_adj back to the value that a CAP_SYS_RESOURCE thread set it to
or its inherited value at fork.  Assuming the thread that has forked it
has oom_score_adj of 0, each process could decrease it back from 0 upon
activation unless a CAP_SYS_RESOURCE thread elevated it to something
higher.

Alternative considered:

* a setuid binary
* a daemon with CAP_SYS_RESOURCE

Since you don't wan't all processes to be able to reduce their oom_adj, a
setuid or daemon implementation would be complex.  The alternatives also
have much higher overhead.

This patch updated from original patch based on feedback from David
Rientjes.
Signed-off-by: Mandeep Singh Baines <msb@chromium.org>
Acked-by: David Rientjes <rientjes@google.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Ying Han <yinghan@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/filesystems/proc.txt

@@ -1323,6 +1323,10 @@ scaled linearly with /proc/<pid>/oom_score_adj.
 Writing to /proc/<pid>/oom_score_adj or /proc/<pid>/oom_adj will change the
 other with its scaled value.
 
+The value of /proc/<pid>/oom_score_adj may be reduced no lower than the last
+value set by a CAP_SYS_RESOURCE process. To reduce the value any lower
+requires CAP_SYS_RESOURCE.
+
 NOTICE: /proc/<pid>/oom_adj is deprecated and will be removed, please see
 Documentation/feature-removal-schedule.txt.
 

fs/proc/base.c

@@ -1151,7 +1151,7 @@ static ssize_t oom_score_adj_write(struct file *file, const char __user *buf,
 		goto err_task_lock;
 	}
 
-	if (oom_score_adj < task->signal->oom_score_adj &&
+	if (oom_score_adj < task->signal->oom_score_adj_min &&
 			!capable(CAP_SYS_RESOURCE)) {
 		err = -EACCES;
 		goto err_sighand;
@@ -1164,6 +1164,8 @@ static ssize_t oom_score_adj_write(struct file *file, const char __user *buf,
 			atomic_dec(&task->mm->oom_disable_count);
 	}
 	task->signal->oom_score_adj = oom_score_adj;
+	if (has_capability_noaudit(current, CAP_SYS_RESOURCE))
+		task->signal->oom_score_adj_min = oom_score_adj;
 	/*
 	 * Scale /proc/pid/oom_adj appropriately ensuring that OOM_DISABLE is
 	 * always attainable.

include/linux/sched.h

@@ -634,6 +634,8 @@ struct signal_struct {
 
 	int oom_adj;		/* OOM kill score adjustment (bit shift) */
 	int oom_score_adj;	/* OOM kill score adjustment */
+	int oom_score_adj_min;	/* OOM kill score adjustment minimum value.
+				 * Only settable by CAP_SYS_RESOURCE. */
 
 	struct mutex cred_guard_mutex;	/* guard against foreign influences on
 					 * credential calculations

kernel/fork.c

@@ -910,6 +910,7 @@ static int copy_signal(unsigned long clone_flags, struct task_struct *tsk)
 
 	sig->oom_adj = current->signal->oom_adj;
 	sig->oom_score_adj = current->signal->oom_score_adj;
+	sig->oom_score_adj_min = current->signal->oom_score_adj_min;
 
 	mutex_init(&sig->cred_guard_mutex);