[PATCH v3] oom: allow a non-CAP_SYS_RESOURCE proces to oom_score_adj down

[Mandeep Singh Baines <msb@chromium.org>]

We'd like to be able to oom_score_adj a process up/down as its
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 or its inherited value at fork. Assuming the thread that has forked
it has oom_score_adj of 0, each tab 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 <rientjes@google.com>.

Signed-off-by: Mandeep Singh Baines <msb@chromium.org>
Acked-by: David Rientjes <rientjes@google.com>
---
 Documentation/filesystems/proc.txt |    4 ++++
 fs/proc/base.c                     |    4 +++-
 include/linux/sched.h              |    2 ++
 kernel/fork.c                      |    1 +
 4 files changed, 10 insertions(+), 1 deletions(-)

diff --git a/Documentation/filesystems/proc.txt b/Documentation/filesystems/proc.txt
index e73df27..7139c50 100644
--- a/Documentation/filesystems/proc.txt
+++ b/Documentation/filesystems/proc.txt
@@ -1296,6 +1296,10 @@ 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.
 
diff --git a/fs/proc/base.c b/fs/proc/base.c
index f3d02ca..7b1a9df 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -1164,7 +1164,7 @@ 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;
@@ -1177,6 +1177,8 @@ 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.
diff --git a/include/linux/sched.h b/include/linux/sched.h
index f53cdf2..2a71ee0 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -626,6 +626,8 @@ 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
diff --git a/kernel/fork.c b/kernel/fork.c
index 3b159c5..0979527 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -907,6 +907,7 @@ 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);
 
-- 
1.7.3.1