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>

1 files changed, 2 insertions, 0 deletions

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 07402530fc70..f23b5bb6f52e 100644
--- a/include/linux/sched.h
+++ b/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