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Subject[PATCH] oom: create a resource limit for oom_adj
For ChromiumOS, we'd like to be able to oom_adj a process up/down
as its leaves/enters the foreground. Currently, it is not possible
to oom_adj down without CAP_SYS_RESOURCE. This patch creates a new
resource limit, RLIMIT_OOMADJ, which is works in a similar fashion
to RLIMIT_NICE. This allows a process's oom_adj to be lowered
without CAP_SYS_RESOURCE as long as the new value is greater
than the resource limit.

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.

Signed-off-by: Mandeep Singh Baines <msb@chromium.org>
---
fs/proc/base.c | 12 ++++++++++--
include/asm-generic/resource.h | 5 ++++-
2 files changed, 14 insertions(+), 3 deletions(-)
diff --git a/fs/proc/base.c b/fs/proc/base.c
index f3d02ca..4384013 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -462,6 +462,7 @@ static const struct limit_names lnames[RLIM_NLIMITS] = {
[RLIMIT_NICE] = {"Max nice priority", NULL},
[RLIMIT_RTPRIO] = {"Max realtime priority", NULL},
[RLIMIT_RTTIME] = {"Max realtime timeout", "us"},
+ [RLIMIT_OOMADJ] = {"Max OOM adjust", NULL},
};

/* Display limits for a process */
@@ -1057,8 +1058,15 @@ static ssize_t oom_adjust_write(struct file *file, const char __user *buf,
}

if (oom_adjust < task->signal->oom_adj && !capable(CAP_SYS_RESOURCE)) {
- err = -EACCES;
- goto err_sighand;
+ /* convert oom_adj [15,-17] to rlimit style value [1,33] */
+ long oom_rlim = OOM_ADJUST_MAX + 1 - oom_adjust;
+
+ if (oom_rlim > task->signal->rlim[RLIMIT_OOMADJ].rlim_cur) {
+ unlock_task_sighand(task, &flags);
+ put_task_struct(task);
+ err = -EACCES;
+ goto err_sighand;
+ }
}

if (oom_adjust != task->signal->oom_adj) {
diff --git a/include/asm-generic/resource.h b/include/asm-generic/resource.h
index 587566f..a8640a4 100644
--- a/include/asm-generic/resource.h
+++ b/include/asm-generic/resource.h
@@ -45,7 +45,9 @@
0-39 for nice level 19 .. -20 */
#define RLIMIT_RTPRIO 14 /* maximum realtime priority */
#define RLIMIT_RTTIME 15 /* timeout for RT tasks in us */
-#define RLIM_NLIMITS 16
+#define RLIMIT_OOMADJ 16 /* max oom_adj allowed to lower to
+ 0-32 for oom level 15 .. -17 */
+#define RLIM_NLIMITS 17

/*
* SuS says limits have to be unsigned.
@@ -86,6 +88,7 @@
[RLIMIT_MSGQUEUE] = { MQ_BYTES_MAX, MQ_BYTES_MAX }, \
[RLIMIT_NICE] = { 0, 0 }, \
[RLIMIT_RTPRIO] = { 0, 0 }, \
+ [RLIMIT_OOMADJ] = { 0, 0 }, \
[RLIMIT_RTTIME] = { RLIM_INFINITY, RLIM_INFINITY }, \
}

--
1.7.3.1


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