Re: xen/gntdev: add ioctl for grant copy

[David Vrabel <david.vrabel@citrix.com>]

On 13/01/16 19:56, Dan Carpenter wrote:
> Hello David Vrabel,
> 
> The patch a4cdb556cae0: "xen/gntdev: add ioctl for grant copy" from
> Dec 2, 2014, leads to the following static checker warning:
> 
> 	drivers/xen/gntdev.c:775 gntdev_get_page()
> 	warn: mask and shift to zero
> 
> drivers/xen/gntdev.c
>    761  static int gntdev_get_page(struct gntdev_copy_batch *batch, void __user *virt,
>    762                             bool writeable, unsigned long *gfn)
>    763  {
>    764          unsigned long addr = (unsigned long)virt;
>    765          struct page *page;
>    766          unsigned long xen_pfn;
>    767          int ret;
>    768  
>    769          ret = get_user_pages_fast(addr, 1, writeable, &page);
>    770          if (ret < 0)
>    771                  return ret;
>    772  
>    773          batch->pages[batch->nr_pages++] = page;
>    774  
>    775          xen_pfn = page_to_xen_pfn(page) + XEN_PFN_DOWN(addr & ~PAGE_MASK);
>                                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> This is zero most of the time.  Did you intend the bitwise negate?

This is non-zero if XEN_PAGE_SIZE < PAGE_SIZE and is needed for this case.

David