[PATCH v1 06/14] mm: introduce bpf_out_of_memory() bpf kfunc

[Roman Gushchin <roman.gushchin@linux.dev>]

Introduce bpf_out_of_memory() bpf kfunc, which allows to declare
an out of memory events and trigger the corresponding kernel OOM
handling mechanism.

It takes a trusted memcg pointer (or NULL for system-wide OOMs)
as an argument, as well as the page order.

If the wait_on_oom_lock argument is not set, only one OOM can be
declared and handled in the system at once, so if the function is
called in parallel to another OOM handling, it bails out with -EBUSY.
This mode is suited for global OOM's: any concurrent OOMs will likely
do the job and release some memory. In a blocking mode (which is
suited for memcg OOMs) the execution will wait on the oom_lock mutex.

The function is declared as sleepable. It guarantees that it won't
be called from an atomic context. It's required by the OOM handling
code, which is not guaranteed to work in a non-blocking context.

Handling of a memcg OOM almost always requires taking of the
css_set_lock spinlock. The fact that bpf_out_of_memory() is sleepable
also guarantees that it can't be called with acquired css_set_lock,
so the kernel can't deadlock on it.

Signed-off-by: Roman Gushchin <roman.gushchin@linux.dev>
---
 mm/oom_kill.c | 45 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 45 insertions(+)

diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 25fc5e744e27..df409f0fac45 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -1324,10 +1324,55 @@ __bpf_kfunc int bpf_oom_kill_process(struct oom_control *oc,
 	return 0;
 }
 
+/**
+ * bpf_out_of_memory - declare Out Of Memory state and invoke OOM killer
+ * @memcg__nullable: memcg or NULL for system-wide OOMs
+ * @order: order of page which wasn't allocated
+ * @wait_on_oom_lock: if true, block on oom_lock
+ * @constraint_text__nullable: custom constraint description for the OOM report
+ *
+ * Declares the Out Of Memory state and invokes the OOM killer.
+ *
+ * OOM handlers are synchronized using the oom_lock mutex. If wait_on_oom_lock
+ * is true, the function will wait on it. Otherwise it bails out with -EBUSY
+ * if oom_lock is contended.
+ *
+ * Generally it's advised to pass wait_on_oom_lock=true for global OOMs
+ * and wait_on_oom_lock=false for memcg-scoped OOMs.
+ *
+ * Returns 1 if the forward progress was achieved and some memory was freed.
+ * Returns a negative value if an error has been occurred.
+ */
+__bpf_kfunc int bpf_out_of_memory(struct mem_cgroup *memcg__nullable,
+				  int order, bool wait_on_oom_lock)
+{
+	struct oom_control oc = {
+		.memcg = memcg__nullable,
+		.order = order,
+	};
+	int ret;
+
+	if (oc.order < 0 || oc.order > MAX_PAGE_ORDER)
+		return -EINVAL;
+
+	if (wait_on_oom_lock) {
+		ret = mutex_lock_killable(&oom_lock);
+		if (ret)
+			return ret;
+	} else if (!mutex_trylock(&oom_lock))
+		return -EBUSY;
+
+	ret = out_of_memory(&oc);
+
+	mutex_unlock(&oom_lock);
+	return ret;
+}
+
 __bpf_kfunc_end_defs();
 
 BTF_KFUNCS_START(bpf_oom_kfuncs)
 BTF_ID_FLAGS(func, bpf_oom_kill_process, KF_SLEEPABLE | KF_TRUSTED_ARGS)
+BTF_ID_FLAGS(func, bpf_out_of_memory, KF_SLEEPABLE | KF_TRUSTED_ARGS)
 BTF_KFUNCS_END(bpf_oom_kfuncs)
 
 static const struct btf_kfunc_id_set bpf_oom_kfunc_set = {
-- 
2.50.1