Re: [PATCH] Fix left over EFI cache mapping problems

[Linus Torvalds]

On Fri, 15 Feb 2008, Huang, Ying wrote:
> 
> I think the patch following may be better, because it is possible that
> the EFI_PAGE_SHIFT and PAGE_SHIFT are different.

If this is a problem in practice, we'd be better off having a helper 
function to do it, to avoid overflows. Right now, doing

> +		unsigned long num_pages;
> +		num_pages = (md->num_pages << EFI_PAGE_SHIFT) >> PAGE_SHIFT;

overflows at 4GB on x86-32. And maybe you never have areas that big, and 
people are moving over to 64-bit anyway, it still sounds like a bug 
waiting to happen.

So *if* we care (I doubt we do, since EFI_PAGE_SHIFT at least right now 
matches PAGE_SHIFT on x86), you'd probably want to do something like

  static inline unsigned long efi_pages_to_native_pages(unsigned long efi_pages)
  {
  #if EFI_PAGE_SHIFT > PAGE_SHIFT
	return efi_pages << (EFI_PAGE_SHIFT - PAGE_SHIFT);
  #else
	return efi_pages >> (PAGE_SHIFT - EFI_PAGE_SHIFT);
  #endif
  }

or whatever.

Otherwise, trying to avoid a bug with different page sizes is actually 
more likely to *introduce* one rather than fix one..

			Linus
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