Messages in this thread | | | Date | Tue, 30 Jun 2009 07:48:17 -0700 | From | "H. Peter Anvin" <> | Subject | Re: [BUG 2.6.31-rc1] HIGHMEM64G causes hang in PCI init on 32-bit x86 |
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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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