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, 3 insertions, 1 deletions

diff --git a/fs/proc/base.c b/fs/proc/base.c
index 93f1cdd5d3d7..9d096e82b201 100644
--- a/fs/proc/base.c
+++ b/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.