Free paging slab space used in spawning a child process

include/aos/paging.h

@@ -135,4 +135,12 @@ static inline lvaddr_t paging_genvaddr_to_lvaddr(genvaddr_t genvaddr) {
 // NOTE rueegges: added debug helper
 void pt_print_state(struct paging_state *st);
 
+/**
+ * @brief Destructive action! NOT THREAD SAFE
+ * This frees all the slab space occupied by the given paging state without performing any unmapping operations
+ * 
+ * @param st paging state to perform destructive action on
+ */
+void paging_free_slabs(struct paging_state *st);
+
 #endif // LIBBARRELFISH_PAGING_H

lib/aos/paging.c

@@ -1049,3 +1049,31 @@ errval_t paging_unmap(struct paging_state *st, const void *region)
 
     return err;
 }
+
+/**
+ * @brief Function to recursively free all the slab space. NOT THREAD SAFE
+ * 
+ * @param pt page table to recursively free the slab space for
+ * @return errval_t 
+ */
+static void paging_free_page_table_tree(struct paging_state *st, struct pt_t *pt) {
+    if(pt == NULL) return;
+    if(pt->children != NULL) {
+        for(size_t i = 0; i < PTABLE_ENTRIES; ++i) {
+            paging_free_page_table_tree(st, pt->children[i]);
+        }
+        slab_free(&st->pt_children_slabs, pt->children);
+    }
+    slab_free(&st->pt_slabs, pt);
+}
+
+void paging_free_slabs(struct paging_state *st) {
+    // free the slab space used by the child paging state so it can be reused by the parent
+    for(size_t i = 0; i < PTABLE_ENTRIES; ++i){
+        paging_free_page_table_tree(st, st->l0_pt.children[i]);
+    }
+    for(struct pt_vaddr_reg_t *reg = st->vaddr_head.next; reg != NULL; reg = reg->next){
+        slab_free(&st->pt_slabs, reg);
+    }
+
+}

lib/spawn/spawn.c

@@ -511,6 +511,9 @@ errval_t spawn_load_argv(int argc, char *argv[], struct spawninfo *si,
     struct paging_state *child_paging_state = &_child_paging_state;
     // 64 * 1024 is enough to catch null pointers and also enough to not conflict with child starting paging_alloc at VADDR_OFFSET
     err = paging_init_state_foreign(child_paging_state, VADDR_LOWEST_NON_NULL, si->vspace_cap_l0_pagetable, get_default_slot_allocator());
+    // we use the parent's slab allocators because we want to later free the child paging state's memory
+    child_paging_state->pt_slabs = get_current_paging_state()->pt_slabs;
+    child_paging_state->pt_children_slabs = get_current_paging_state()->pt_children_slabs;
     if (err_is_fail(err)) return err_push(err, SPAWN_ERR_PAGING_INIT);
 
     // - Load the ELF binary
@@ -641,6 +644,7 @@ errval_t spawn_load_argv(int argc, char *argv[], struct spawninfo *si,
     if (err_is_fail(err)) {
         return err_push(err, SPAWN_ERR_SERIALISE_VSPACE);
     }
+    paging_free_slabs(child_paging_state);
     // so we don't use it after serializing
     child_paging_state = NULL;