kvmalloc函數

你應該曾經糾結過是用kmalloc()，仍是vmalloc()？如今你不用那麼糾結了，由於內核裏面如今有個API叫kvmalloc()，能夠認爲是kmalloc()和vmalloc()的雙劍合一。屠龍刀和倚天劍的合體。

內核裏面有大量的代碼如今都使用了kvmalloc()，譬如：

source/ipc/msg.c

static int newque(struct ipc_namespace *ns, struct ipc_params *params)
{
  struct msg_queue *msq;
  int retval;
  key_t key = params->key;
  int msgflg = params->flg;

  msq = kvmalloc(sizeof(*msq), GFP_KERNEL);
  if (unlikely(!msq))
    return -ENOMEM;

  ...
}

這個代碼在早期的內核裏面是（好比v4.0-rc7/source/ipc/msg.c）：

static int newque(struct ipc_namespace *ns, struct ipc_params *params)
{
  struct msg_queue *msq;
  int id, retval;
  key_t key = params->key;
  int msgflg = params->flg;

  msq = ipc_rcu_alloc(sizeof(*msq));
  if (!msq)
    return -ENOMEM;

 ...

}

看起來是用的這個函數申請內存：

ipc_rcu_alloc(sizeof(*msq))

那麼這個ipc_rc_alloc()是怎麼回事呢？

void *ipc_alloc(int size)
{
  void *out;
  if (size > PAGE_SIZE)
    out = vmalloc(size);
  else
    out = kmalloc(size, GFP_KERNEL);
  return out;
}

邏輯上是，大於一頁的時候用vmalloc()，小於等於1頁用kmalloc()。

而kvmalloc()的實現代碼裏面則對相似邏輯進行了很是智能地處理：

void *kvmalloc_node(size_t size, gfp_t flags, int node)
{
  gfp_t kmalloc_flags = flags;
  void *ret;

  /*
   * vmalloc uses GFP_KERNEL for some internal allocations (e.g page tables)
   * so the given set of flags has to be compatible.
   */
  if ((flags & GFP_KERNEL) != GFP_KERNEL)
    return kmalloc_node(size, flags, node);

  /*
   * We want to attempt a large physically contiguous block first because
   * it is less likely to fragment multiple larger blocks and therefore
   * contribute to a long term fragmentation less than vmalloc fallback.
   * However make sure that larger requests are not too disruptive - no
   * OOM killer and no allocation failure warnings as we have a fallback.
   */
  if (size > PAGE_SIZE) {
    kmalloc_flags |= __GFP_NOWARN;

    if (!(kmalloc_flags & __GFP_RETRY_MAYFAIL))
      kmalloc_flags |= __GFP_NORETRY;
  }

  ret = kmalloc_node(size, kmalloc_flags, node);

  /*
   * It doesn't really make sense to fallback to vmalloc for sub page
   * requests
   */
  if (ret || size <= PAGE_SIZE)
    return ret;

  return __vmalloc_node_flags_caller(size, node, flags,
      __builtin_return_address(0));
}
EXPORT_SYMBOL(kvmalloc_node);

static inline void *kvmalloc(size_t size, gfp_t flags)
{
  return kvmalloc_node(size, flags, NUMA_NO_NODE);
}

大於一個page的時候，會先用kmalloc()進行__GFP_NORETRY的嘗試，若是嘗試失敗就fallback到vmalloc（NORETRY標記避免了kmalloc在申請內存失敗地狀況下，反覆嘗試甚至作OOM來得到內存）。

固然，kvmalloc()的size若是小於1個page，則沿用老的kmalloc()邏輯，並且也不會設置__GFP_NORETRY，若是反覆嘗試失敗的話，也不會fallback到vmalloc()，由於vmalloc()申請小於1個page的內存是不合適的。