Re: HIGHMEM4G config for 1GB RAM on desktop?

Re: HIGHMEM4G config for 1GB RAM on desktop?

[linux]

[Resend because my fingers STILL type "@vger.rutgers.edu" if I'm not
paying attention...]

> In practice, I suspect 2/2 will do exactly what you want ... and what 
> 99.9% of people want actually ;-) We could add more options, but be sure
> to mark anything that's not 1GB aligned as not suitable for PAE (as the
> 0.5 split was).

But if you're using PAE, you've got > 4G of RAM, so there's no need to
be clever trying to avoid HIGHMEM options.

Unfortunately, I just had a server with Con's patch detonate overnight
(Oops in interrupt -> panic; details in separate e-mail), and wli tells
me that there are additional places in the code that need fixing.

I notice that all previous patches had the kernel range a power of 2 in size.
Is this required somewhere?  I thought it was just that the kernel
had to start at a PGD boundary (4M on normal x86, 1G on PAE).

If 128M is always enough, a split at 0xb800000 seems possible, but
giving it an extra 128M seems like a nice bit of safety for PCI devices.

[PATCH] (was Re: HIGHMEM4G config for 1GB RAM on desktop?)

[Roland Dreier]

    > wli tells me that there are additional places in the code that
    > need fixing.

Looks like arch/i386/kernel/doublefault.c is one place in the code
that hardcodes the assumption that PAGE_OFFSET == 0xC0000000.  Here's
a patch that fixes that.

(Of course this doesn't really fix anything except debugging output)

 - Roland

Signed-off-by: Roland Dreier <roland@topspin.com>

Index: linux-2.6.8-rc1/arch/i386/kernel/doublefault.c
===================================================================
--- linux-2.6.8-rc1.orig/arch/i386/kernel/doublefault.c
+++ linux-2.6.8-rc1/arch/i386/kernel/doublefault.c
@@ -13,7 +13,7 @@
 static unsigned long doublefault_stack[DOUBLEFAULT_STACKSIZE];
 #define STACK_START (unsigned long)(doublefault_stack+DOUBLEFAULT_STACKSIZE)
 
-#define ptr_ok(x) ((x) > 0xc0000000 && (x) < 0xc1000000)
+#define ptr_ok(x) ((x) > PAGE_OFFSET && (x) < PAGE_OFFSET + 0x1000000)
 
 static void doublefault_fn(void)
 {

Re: HIGHMEM4G config for 1GB RAM on desktop?

[Valdis.Kletnieks]

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On Fri, 06 Aug 2004 12:52:36 -0000, linux@horizon.com said:

> But if you're using PAE, you've got > 4G of RAM, so there's no need to
> be clever trying to avoid HIGHMEM options.

Poking around in arch/i386/mm/init.c finds this:

#ifdef CONFIG_X86_PAE
int nx_enabled = 0;

static void __init set_nx(void)
{
        unsigned int v[4], l, h;

        if (cpu_has_pae && (cpuid_eax(0x80000000) > 0x80000001)) {

I could certainly see somebody wanting to use the NX stuff with only 2G of RAM.....

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