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

H. Peter Anvin
2009-06-30T14:50:55Z

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.