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    SubjectRe: [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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