Introduce bpf_oom_kill_process() bpf kfunc, which is supposed
to be used by bpf OOM programs. It allows to kill a process
in exactly the same way the OOM killer does: using the OOM reaper,
bumping corresponding memcg and global statistics, respecting
memory.oom.group etc.

On success, it sets om_control's bpf_memory_freed field to true,
enabling the bpf program to bypass the kernel OOM killer.

Signed-off-by: Roman Gushchin <roman.gushchin@linux.dev>
---
 mm/oom_kill.c | 50 ++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 50 insertions(+)

diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index d00776b63c0a..2e922e75a9df 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -1299,6 +1299,42 @@ SYSCALL_DEFINE2(process_mrelease, int, pidfd, unsigned int, flags)
 
 #ifdef CONFIG_BPF_SYSCALL
 
+__bpf_kfunc_start_defs();
+/*
+ * Kill a process in a way similar to the kernel OOM killer.
+ * This means dump the necessary information to dmesg, adjust memcg
+ * statistics, leverage the oom reaper, respect memory.oom.group etc.
+ *
+ * bpf_oom_kill_process() marks the forward progress by setting
+ * oc->bpf_memory_freed. If the progress was made, the bpf program
+ * is free to decide if the kernel oom killer should be invoked.
+ * Otherwise it's enforced, so that a bad bpf program can't
+ * deadlock the machine on memory.
+ */
+__bpf_kfunc int bpf_oom_kill_process(struct oom_control *oc,
+				     struct task_struct *task,
+				     const char *message__str)
+{
+	if (oom_unkillable_task(task))
+		return -EPERM;
+
+	/* paired with put_task_struct() in oom_kill_process() */
+	task = tryget_task_struct(task);
+	if (!task)
+		return -EINVAL;
+
+	oc->chosen = task;
+
+	oom_kill_process(oc, message__str);
+
+	oc->chosen = NULL;
+	oc->bpf_memory_freed = true;
+
+	return 0;
+}
+
+__bpf_kfunc_end_defs();
+
 __bpf_hook_start();
 
 /*
@@ -1319,6 +1355,16 @@ static const struct btf_kfunc_id_set bpf_oom_hook_set = {
 	.owner = THIS_MODULE,
 	.set   = &bpf_oom_hooks,
 };
+
+BTF_KFUNCS_START(bpf_oom_kfuncs)
+BTF_ID_FLAGS(func, bpf_oom_kill_process, KF_SLEEPABLE | KF_TRUSTED_ARGS)
+BTF_KFUNCS_END(bpf_oom_kfuncs)
+
+static const struct btf_kfunc_id_set bpf_oom_kfunc_set = {
+	.owner          = THIS_MODULE,
+	.set            = &bpf_oom_kfuncs,
+};
+
 static int __init bpf_oom_init(void)
 {
 	int err;
@@ -1326,6 +1372,10 @@ static int __init bpf_oom_init(void)
 	err = register_btf_fmodret_id_set(&bpf_oom_hook_set);
 	if (err)
 		pr_warn("error while registering bpf oom hooks: %d", err);
+	err = register_btf_kfunc_id_set(BPF_PROG_TYPE_TRACING,
+					&bpf_oom_kfunc_set);
+	if (err)
+		pr_warn("error while registering bpf oom kfuncs: %d", err);
 
 	return err;
 }
-- 
2.49.0.901.g37484f566f-goog