Fixes allocator heap corruption bug and assertion failure.

- page_collect shouldn't assume that the page has an empty free list,
  but neither should it do O(n) work when it does not.
- After page_free, alloc_generic should not refer to the page anymore.

Signed-off-by: Nathan Ringo <me@remexre.com>

Author: remexre

src/kernel/mm/alloc.c

@@ -111,12 +111,14 @@ void *alloc_generic(usize size, struct mm_alloc_heap *heap) {
     page = container_of(page_head, struct mm_alloc_page, list);
     page_head = page_head->next;
 
-    // Collect free objects in the page.
-    page_collect(page);
-
     // Free the page if it's empty.
-    if (page_is_empty(page))
+    if (page_is_empty(page)) {
       page_free(page, heap);
+      continue;
+    }
+
+    // Collect free objects in the page.
+    page_collect(page);
 
     // If there are any free objects, use them.
     if (!page_is_full(page))

src/kernel/mm/alloc/page.c

@@ -271,8 +271,11 @@ void page_bounds(const struct mm_alloc_page *page, uptr *out_start,
 }
 
 void page_collect(struct mm_alloc_page *page) {
+  // If we can allocate an object from the page, don't bother collecting it.
+  if (block_ref_deref(page, page->free))
+    return;
+
   // Since our hart owns the local free list, we can just move it over.
-  assert(!block_ref_deref(page, page->free));
   page->free = page->local_free;
   page->local_free = block_ref_make(page, nullptr);