Somewhere after the 2.6.18 timeframe, Andi Kleen made the
x86_64 e820 map __initdata:
struct e820map e820 __initdata;
This is fine for upstream x86_64 kernels, because the e820 map never
gets used during runtime. But because we (RHEL/Fedora) have an x86_64
version of page_is_ram(), it ends up using __init data.
I became aware of this when I got an FC6/crash-utility bugzilla #237383,
filed because the crash utility started failing on later FC6 live systems.
This is because the kernel's /dev/crash module crash.ko started failing
because it uses page_is_ram() as a memory access qualifer.
Also, because of Red Hat's restriction on the use of /dev/mem to only
the first 256 RAM pages, the __initdata addition ends up opening the flood
gates for /dev/mem usage. That's because of this:
/*
* devmem_is_allowed() checks to see if /dev/mem access to a certain address is
* valid. The argument is a physical page number.
*
*
* On x86-64, access has to be given to the first megabyte of ram because that area
* contains bios code and data regions used by X and dosemu and similar apps.
* Access has to be given to non-kernel-ram areas as well, these contain the PCI
* mmio resources as well as potential bios/acpi data regions.
*/
int devmem_is_allowed(unsigned long pagenr)
{
if (pagenr <= 256)
return 1;
if (!page_is_ram(pagenr))
return 1;
return 0;
}
The function is meant to allow /dev/mem accesses above pagenr 256
only if they are *not* RAM -- but since since page_is_ram() is failing,
it inadvertantly allows access to any pagenr.
(Interestingly enough, this bug allows the user of the crash
utility to work around the /dev/crash failure by alternatively
using /dev/mem instead! /dev/crash was only created to begin
with because of the /dev/mem restriction...
Anyway, reverting back from __initdata fixes the situation for both
/dev/crash and the /dev/mem restriction.
Dave Anderson
(patch is against 2.6.20-1.2944)
--- linux-2.6.20.x86_64/arch/x86_64/kernel/e820.c.orig 2007-04-26 14:38:12.000000000 -0400
+++ linux-2.6.20.x86_64/arch/x86_64/kernel/e820.c 2007-04-26 14:38:24.000000000 -0400
@@ -25,7 +25,7 @@
#include <asm/bootsetup.h>
#include <asm/sections.h>
-struct e820map e820 __initdata;
+struct e820map e820;
/*
* PFN of last memory page.
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