MT#55283 delegate shm refcount to table

We don't track individual uses of the shared memory, so we can't safely
free it when the mapping is closed, as the internal forwarding targets
may still point into it. Delay freeing until the table itself is freed.
Each mmap then counts as another reference on the table. Unmap then
simply decreases the reference count but never actually frees the pages.

Change-Id: Ic33454155cd0083f733711ce52699047cff9e56c
(cherry picked from commit de961d5b43)

kernel-module/xt_RTPENGINE.c

@@ -478,7 +478,6 @@ struct re_hmac {
 };
 
 struct re_shm {
-	atomic_t			users;
 	void				*head;
 	struct rtpengine_table		*table;
 	struct list_head		list_entry;
@@ -1993,31 +1992,20 @@ static void vm_mmap_open(struct vm_area_struct *vma) {
 	if (!(shm = vma->vm_private_data))
 		return;
 
-	atomic_inc(&shm->users);
+	ref_get(shm->table);
 }
 
 static void vm_mmap_close(struct vm_area_struct *vma) {
 	struct re_shm *shm;
-	struct rtpengine_table *t;
 
 	if (vma->vm_ops != &vm_mmap_ops)
 		return;
 	if (!(shm = vma->vm_private_data))
 		return;
 
-	if (!atomic_dec_and_test(&shm->users))
-		return;
-
-	t = shm->table;
-
-	spin_lock(&t->shm_lock);
-	list_del_init(&shm->list_entry);
-	spin_unlock(&t->shm_lock);
-
-	vfree(shm->head);
-	kfree(shm);
-
 	vma->vm_private_data = NULL;
+
+	table_put(shm->table);
 }
 
 static void *shm_map_resolve(void *p, size_t size) {
@@ -2885,8 +2873,7 @@ static int proc_control_mmap(struct file *file, struct vm_area_struct *vma) {
 	}
 
 	shm->head = pages;
-	atomic_set(&shm->users, 1);
-	shm->table = t;
+	shm->table = t; // not a reference
 
 	vma->vm_private_data = shm;
 	vma->vm_ops = &vm_mmap_ops;
@@ -2910,7 +2897,7 @@ static int proc_control_mmap(struct file *file, struct vm_area_struct *vma) {
 	list_add(&shm->list_entry, &t->shm_list);
 	spin_unlock(&t->shm_lock);
 
-	table_put(t);
+	// retain reference on table - belongs to the shm list now
 
 	return 0;
 }