Hi Jason,

I love your patch! Yet something to improve:

[auto build test ERROR on net-next/master]

url:    https://github.com/0day-ci/linux/commits/Jason-Wang/vhost-accelerate-metadata-access-through-vmap/20181214-200417
config: mips-malta_kvm_defconfig (attached as .config)
compiler: mipsel-linux-gnu-gcc (Debian 7.2.0-11) 7.2.0
reproduce:
        wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
        chmod +x ~/bin/make.cross
        # save the attached .config to linux build tree
        GCC_VERSION=7.2.0 make.cross ARCH=mips 

All errors (new ones prefixed by >>):

   drivers//vhost/vhost.c: In function 'vhost_init_vmap':
>> drivers//vhost/vhost.c:648:3: error: implicit declaration of function 'release_pages'; did you mean 'release_task'? [-Werror=implicit-function-declaration]
      release_pages(pages, npinned);
      ^~~~~~~~~~~~~
      release_task
   cc1: some warnings being treated as errors

vim +648 drivers//vhost/vhost.c

   619	
   620	static int vhost_init_vmap(struct vhost_vmap *map, unsigned long uaddr,
   621				   size_t size, int write)
   622	{
   623		struct page **pages;
   624		int npages = DIV_ROUND_UP(size, PAGE_SIZE);
   625		int npinned;
   626		void *vaddr;
   627	
   628		pages = kmalloc_array(npages, sizeof(struct page *), GFP_KERNEL);
   629		if (!pages)
   630			return -ENOMEM;
   631	
   632		npinned = get_user_pages_fast(uaddr, npages, write, pages);
   633		if (npinned != npages)
   634			goto err;
   635	
   636		vaddr = vmap(pages, npages, VM_MAP, PAGE_KERNEL);
   637		if (!vaddr)
   638			goto err;
   639	
   640		map->pages = pages;
   641		map->addr = vaddr + (uaddr & (PAGE_SIZE - 1));
   642		map->npages = npages;
   643	
   644		return 0;
   645	
   646	err:
   647		if (npinned > 0)
 > 648			release_pages(pages, npinned);
   649		kfree(pages);
   650		return -EFAULT;
   651	}
   652	

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