Hello all.

I have been having problems using ioremap and iounmap on systems with more
than 1Gb of memory.
My current machine has an athlon processor and 1.5Gb of memory.

I created a driver with the following code:

#define MAXHM 0x1000000 // 16 Mb
int init_module(void)
{
    void *kHighMem;

    printk(KERN_ERR"before ioremap\n");
    kHighMem = ioremap(__pa(high_memory), MAXHM);
    printk(KERN_ERR"ioremap ret: %x\n",kHighMem);
    iounmap(kHighMem);
    printk(KERN_ERR"iounmap passed\n");

    return 0;
}

void cleanup_module(void) {
    printk(KERN_ERR,"module successfully unloaded.\n");
}

I have compiled the code into a module named "test.o" and have added the
append="mem=1280" to lilo for both kernels.
Under 2.4.18:
If I run "insmod test.o" , I get a Segmentation fault (see attached messages
file).

Under 2.4.20:
 I can run "insmod test.o" and "rmmod test" without a problem.  But then it
seems if I run any program that accesses memory (a simple user mode program
that malloc's multiple buffers for example) I get a Kernel Panic and the
computer locks up.  The panic I get is:
Unable to handle kernel NULL pointer dereference at virtual address 00000004
 printing eip:
c012ca6e
*pde = 00000000
Oops: 0002
CPU:    0
EIP:    0010:[<c012ca6e>]    Not tainted
EFLAGS: 00010092
eax:   ebx:   ecx:    edx: esi:   edi:   ebp:    esp:  ds:  es:  ss: ...
Process init (pid: 521, stackpage=f6ed1000)
Stack: ...
Call Trace:    ...
Code: 89 50 04 89 02 c7 43 04 00 00 00 00 c7 03 00 00 00 00 d1 6c

I have not yet to tried to trace into the 2.4.20 lockup.

If append="mem=768" or less the problem doesn't happen, but ioremap is
unable to map the entire amount of remaining memory above the kernel and a
boundary is reached of kernel memory + ioremaped memory <= 1Gb (i.e. if
append="mem=384" ioremap can only map up to 640mb).

>From what I can guess, it looks like ioremap may be corrupting the page
tables when the highmem functionality is being used.  I noticed that my
PAGE_OFFSET is still at 0xc0000000 which according to <include/page.h> means
that I will always have only 950MB of virtual address space.  I would have
thought that setting CONFIG_HIGHMEM4G=y would have changed the PAGE_OFFSET
value.
Has anyone experienced similar problems?  Does anyone know a different way
of allocating large contiguous amounts of DMA capable memory other than
using ioremap or vmalloc?

Any help would be appreciated.
Robert Murphy