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Message-ID: <alpine.LFD.1.00.0802142139260.4255@woody.linux-foundation.org>
Date: Thu, 14 Feb 2008 21:44:57 -0800 (PST)
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: "Huang, Ying" <ying.huang@...el.com>
cc: Ingo Molnar <mingo@...e.hu>, Andi Kleen <andi@...stfloor.org>,
tglx@...utronix.de, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Fix left over EFI cache mapping problems
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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