linux-kernel - Re: [BUG 2.6.31-rc1] HIGHMEM64G causes hang in PCI init on 32-bit x86

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Message-ID: <4A4A25B1.5010102@zytor.com>
Date:	Tue, 30 Jun 2009 07:48:17 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Mikael Pettersson <mikpe@...uu.se>
CC:	Yinghai Lu <yinghai@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>, Matthew Wilcox <matthew@....cx>,
	Grant Grundler <grundler@...isc-linux.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-pci@...r.kernel.org
Subject: Re: [BUG 2.6.31-rc1] HIGHMEM64G causes hang in PCI init on 32-bit
 x86

Mikael Pettersson wrote:
> 
> Thanks, 2.6.31-rc1 vanilla (which didn't boot) plus this one does boot.
> /proc/iomem now looks as follows:
> 

... as it should.  So far so good, and this is a real problem.

However, there is something that really bothers me: *why does this help
on Mikael's system, which is PAE and therefore has a 64-bit
resource_size_t*?  This whole patch should be a no-op!  There is still
something that doesn't make sense.

The use of "unsigned long" in ram_alignment() will overflow after 2^52
bytes, but again, that's not the issue here, since the highest "start"
value we have is (0x2 << 32).

By process of elimination, the culprit must be round_up(), which reveals
that the macro definition of round_up() has a *very* sublte behavior
with mixed types:

#define round_up(x, y) (((x) + (y) - 1) & ~((y) - 1))

ram_alignment() returns unsigned long, which becomes (y).  This means
that the mask word on the right hand of the & gets truncated to 32 bits
*before* the masking happens -- since ((y) - 1) is still unsigned long,
inverting it will not set bits [63..32] to on.

I think this macro is actively dangerous.  Better would be:

({ __typeof__(x) __mask = (y)-1;  ((x)+__mask) & ~__mask; })

... which is also multiple-inclusion-free at the cost of using gcc
({...}) constructs.

The deep irony in this is that in our particular case is perhaps that
align_up(x,y)-1 is the same thing as x | (y-1) which would have avoided
the problem...

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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