Nuttx之mm_extend

声明：此处代码分析，来源与 nuttx 12.8.0版本。

/****************************************************************************
 * Name: mm_extend
 *
 * Description:
 *   Extend a heap region by add a block of (virtually) contiguous memory
 *   to the end of the heap.
 *
 ****************************************************************************/

void mm_extend(FAR struct mm_heap_s *heap, FAR void *mem, size_t size,
               int region)
{
  FAR struct mm_allocnode_s *oldnode;
  FAR struct mm_allocnode_s *newnode;
  uintptr_t blockstart;
  uintptr_t blockend;

  /* Make sure that we were passed valid parameters */

  DEBUGASSERT(heap && mem);
#if CONFIG_MM_REGIONS > 1
  DEBUGASSERT(size >= MIN_EXTEND && region >= 0 &&
              region < heap->mm_nregions);
#else
  DEBUGASSERT(size >= MIN_EXTEND && region == 0);
#endif

  /* Make sure that the memory region are properly aligned */

  blockstart = (uintptr_t)mem;
  blockend   = blockstart + size;

  DEBUGASSERT(MM_ALIGN_UP(blockstart) == blockstart);
  DEBUGASSERT(MM_ALIGN_DOWN(blockend) == blockend);

  /* Take the memory manager mutex */

  DEBUGVERIFY(mm_lock(heap));

  /* Get the terminal node in the old heap.  The block to extend must
   * immediately follow this node.
   */

  oldnode = heap->mm_heapend[region];
  DEBUGASSERT((uintptr_t)oldnode + MM_SIZEOF_ALLOCNODE == blockstart);

  /* The size of the old node now extends to the new terminal node.
   * This is the old size (MM_SIZEOF_ALLOCNODE) plus the size of
   * the block (size) minus the size of the new terminal node
   * (MM_SIZEOF_ALLOCNODE) or simply:
   */

  oldnode->size = size | (oldnode->size & MM_MASK_BIT);

  /* The old node should already be marked as allocated */

  DEBUGASSERT(MM_NODE_IS_ALLOC(oldnode));

  /* Get and initialize the new terminal node in the heap */

  newnode       = (FAR struct mm_allocnode_s *)
                  (blockend - MM_SIZEOF_ALLOCNODE);
  newnode->size = MM_SIZEOF_ALLOCNODE | MM_ALLOC_BIT;

  heap->mm_heapend[region] = newnode;

  /* Finally, increase the total heap size accordingly */

  heap->mm_heapsize += size;
  mm_unlock(heap);

  /* Finally "free" the new block of memory where the old terminal node was
   * located.
   */

  mm_free(heap, mem);
}

显然，此函数用于扩展堆空间。首先对函数的参数进行合理性检验自然属于基本操作。对于size的检验标准为size>=MIN_EXTEND，那么，问题来了，为何这样的size就是合理的呢？首先我们先来看看MIN_EXTEND的定义。

#define MIN_EXTEND (2 * MM_SIZEOF_ALLOCNODE)

可以看的出来，size大小的空间至少能容纳得下两个struct mm_allocnode_s。众所周知，堆是由struct mm_heap_s来进行管理的。mm_heap_s的成员mm_heapend成员用来标记堆空间的结束，这就解释了MIN_EXTEND中的一个struct mm_allocnode_s结构的来源。我们的函数功能是扩展堆空间，那么，扩展的空间必然是空闲的。对堆空间的扩展实质上就是增加struct mm_heap_s 的mm_nodelist的成员。struct mm_freenode_s用于空闲空间的管理。且，struct mm_freenode_s结构是存在于每个空闲空间的开头。这就解释了MIN_EXTEND中的第二个struct mm_allocnode_s结构的来源。有人说了，struct mm_freenode_s与struct mm_allocnode_s相差两个struct mm_freenode_s这指针的大小，确实，按照严格意义来说，应该修改MIN_EXTEND的定义如下。

#define MIN_EXTEND （sizeof(struct mm_freenode_s) + MM_SIZEOF_ALLOCNODE)

接下来的一个问题是，为什么会有如下代码。

  oldnode->size = size | (oldnode->size & MM_MASK_BIT);

我的意思是，oldnode是属于struct mm_allocnode_s这个类型的，根据mm_addregion我们知道，对于新加入堆的空间struct mm_allocnode_s的size成员变量表示大小的部分，其值一般是MM_SIZEOF_ALLOCNODE,如下。

void mm_addregion(FAR struct mm_heap_s *heap, FAR void *heapstart,
                  size_t heapsize)
{
  ......

  heap->mm_heapstart[idx]->size    = MM_SIZEOF_ALLOCNODE | MM_ALLOC_BIT;
  
  ......

  heap->mm_heapend[idx]->size      = MM_SIZEOF_ALLOCNODE | MM_ALLOC_BIT |
                                     MM_PREVFREE_BIT;

  ......
}

对于此处oldnode->size,其原因是，此时的oldnode,在mm_free间接调用mm_malloc_size时，mm_malloc_size做了强制类型转换。这也印证了我们之前说的，struct mm_freenode_s结构是存在于每个空闲空间的开头。

size_t mm_malloc_size(FAR struct mm_heap_s *heap, FAR void *mem)
{
  ......

  node = (FAR struct mm_freenode_s *)((FAR char *)mem - MM_SIZEOF_ALLOCNODE);
  
  ......
}

那么，struct mm_freenode_s的size成员是表示的同oldnode->size = size 相同的意思吗？即 struct mm_freenode_s的size成员表示的是sizeof(struct mm_freenode_s)+用户可用空间大小吗？显然是的。同样，可以从mm_addregion中就能看出来。

void mm_addregion(FAR struct mm_heap_s *heap, FAR void *heapstart,
                  size_t heapsize)
{
  FAR struct mm_freenode_s *node;

  ......

  node                             = (FAR struct mm_freenode_s *)
                                     (heapbase + MM_SIZEOF_ALLOCNODE);
  DEBUGASSERT((((uintptr_t)node + MM_SIZEOF_ALLOCNODE) % MM_ALIGN) == 0);
  node->size                       = heapsize - 2 * MM_SIZEOF_ALLOCNODE;

  ......
}

mm_addregion对struct mm_allocnode_s的size成员的赋值有一定的特殊性，那么，此size表示的是那段空间呢？更一般的来说，此size表示的是sizeof(struct mm_allocnode_s)+用户实际使用空间大小，何以见得呢？我们可以通过函数mm_malloc就能看得出来。

FAR void *mm_malloc(FAR struct mm_heap_s *heap, size_t size)
{
      FAR struct mm_freenode_s *node;

      ......

      alignsize = MM_ALIGN_UP(size + MM_ALLOCNODE_OVERHEAD);

      ......

      node->size = alignsize | (node->size & MM_MASK_BIT);

      ......


      ret = (FAR void *)((FAR char *)node + MM_SIZEOF_ALLOCNODE);

      ......

}