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

[Mandeep Singh Baines <msb@google.com>]

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 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 based on feedback from
David Rientjes <rientjes@google.com>.

Change-Id: If8f52363fd6c156e1730f43148aee987260e9c72
Signed-off-by: Mandeep Singh Baines <msb@chromium.org>
---
 fs/proc/base.c        |    4 +++-
 include/linux/sched.h |    2 ++
 2 files changed, 5 insertions(+), 1 deletions(-)

diff --git a/fs/proc/base.c b/fs/proc/base.c
index f3d02ca..e617413 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 (capable(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
-- 
1.7.3.1