[RFC PATCH v5 mm-new 3/5] mm: thp: add a new kfunc bpf_mm_get_task()

[Yafang Shao <laoar.shao@gmail.com>]

We will utilize this new kfunc bpf_mm_get_task() to retrieve the
associated task_struct from the given @mm. The obtained task_struct must
be released by calling bpf_task_release() as a paired operation.

Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
---
 mm/bpf_thp.c | 34 ++++++++++++++++++++++++++++++++++
 1 file changed, 34 insertions(+)

diff --git a/mm/bpf_thp.c b/mm/bpf_thp.c
index bdcf6f6af99b..8ed1bf0d7f4d 100644
--- a/mm/bpf_thp.c
+++ b/mm/bpf_thp.c
@@ -205,11 +205,45 @@ __bpf_kfunc void bpf_put_mem_cgroup(struct mem_cgroup *memcg)
 #endif
 }
 
+/**
+ * bpf_mm_get_task - Get the task struct associated with a mm_struct.
+ * @mm: The mm_struct to query
+ *
+ * The obtained task_struct must be released by calling bpf_task_release().
+ *
+ * Return: The associated task_struct on success, or NULL on failure. Note that
+ * this function depends on CONFIG_MEMCG being enabled - it will always return
+ * NULL if CONFIG_MEMCG is not configured.
+ */
+__bpf_kfunc struct task_struct *bpf_mm_get_task(struct mm_struct *mm)
+{
+#ifdef CONFIG_MEMCG
+	struct task_struct *task;
+
+	if (!mm)
+		return NULL;
+	rcu_read_lock();
+	task = rcu_dereference(mm->owner);
+	if (!task)
+		goto out;
+	if (!refcount_inc_not_zero(&task->rcu_users))
+		goto out;
+
+	rcu_read_unlock();
+	return task;
+
+out:
+	rcu_read_unlock();
+#endif
+	return NULL;
+}
+
 __bpf_kfunc_end_defs();
 
 BTF_KFUNCS_START(bpf_thp_ids)
 BTF_ID_FLAGS(func, bpf_mm_get_mem_cgroup, KF_TRUSTED_ARGS | KF_ACQUIRE | KF_RET_NULL)
 BTF_ID_FLAGS(func, bpf_put_mem_cgroup, KF_RELEASE)
+BTF_ID_FLAGS(func, bpf_mm_get_task, KF_TRUSTED_ARGS | KF_ACQUIRE | KF_RET_NULL)
 BTF_KFUNCS_END(bpf_thp_ids)
 
 static const struct btf_kfunc_id_set bpf_thp_set = {
-- 
2.47.3