From: Larry Woodman <lwoodman@redhat.com>
Date: Thu, 2 Oct 2008 12:02:17 -0400
Subject: [x86] PAE: limit RAM to 64GB/PAE36
Message-id: 1222963337.22762.32.camel@dhcp-100-19-198.bos.redhat.com
O-Subject: [RHEL5 patch] Prevent panic on boot by limiting RAM to 64GB/PAE36 on 32-bit/x86 systems
Bugzilla: 465373
RH-Acked-by: Dave Anderson <anderson@redhat.com>

We ran into a strange problem during hardware certification testing
recently.  While performing RHEL4 hardware cert tests in a 96GB x86_64
system it was noticed that the x86/32-bit hugemem kernel boots and runs
successfully with all 96GB of RAM.  Originally x86 RAM sizes were
limited to 36 bits via PAE36 while x86_64 systems supported 40 bits via
PAE40.  Evidently PAE40 works in 32-bit compatibility mode since more
than 64GB of RAM runs fine when an x86_64 system runs an x86 kernel.

------------------------------------------------------------
BIOS-provided physical RAM map:

BIOS-e820: 0000000000000000 - 00000000000a0000 (usable)
BIOS-e820: 0000000000100000 - 00000000cfaa0000 (usable)
BIOS-e820: 00000000cfaa0000 - 00000000cfab6000 (reserved)
BIOS-e820: 00000000cfab6000 - 00000000cfad5c00 (ACPI data)
BIOS-e820: 00000000cfad5c00 - 00000000d0000000 (reserved)
BIOS-e820: 00000000f0000000 - 00000000f8000000 (reserved)
BIOS-e820: 00000000fe000000 - 0000000100000000 (reserved)
BIOS-e820: 0000000100000000 - 0000001830000000 (usable)

98176MB HIGHMEM available.
896MB LOWMEM available.
------------------------------------------------------------

Everything seems to work OK in RHEL4 because the hugemem kernel has
enough lowmem to fit the mem_map of system with this much RAM.  The
problem exists with RHEL5.  Since RHEL5 has no hugemem kernel and
mem_map is too large to fit in lowmem of the PAE kernel, a large memory
x86_64 system will panic when booting the 32-bit PAE kernel.

While this is not a critical issue because we do not officially support
32-bit systems with huge amounts of RAM and its doubtful that customers
will do this, it is a regression from RHEL4 to RHEL5.

A simple fix is to limit the x86 memory size to 64GB by obeying PAE36.

------------------------------------------------------------------------
BIOS-provided physical RAM map:
 BIOS-e820: 0000000000000000 - 000000000009d800 (usable)
 BIOS-e820: 000000000009d800 - 00000000000a0000 (reserved)
 BIOS-e820: 00000000000ce000 - 0000000000100000 (reserved)
 BIOS-e820: 0000000000100000 - 0000000037e70000 (usable)
 BIOS-e820: 0000000037e70000 - 0000000037ed8000 (ACPI data)
 BIOS-e820: 0000000037ed8000 - 0000000037f00000 (ACPI NVS)
 BIOS-e820: 0000000037f00000 - 00000000f0000000 (usable)
 BIOS-e820: 00000000f8000000 - 00000000fec00000 (reserved)
 BIOS-e820: 00000000fffc0000 - 0000000100000000 (reserved)
 BIOS-e820: 0000000100000000 - 0000002008000000 (usable)
RAM exceeds maximum supported memory for x86, Truncating to 64GB
64640MB HIGHMEM available.
896MB LOWMEM available.
------------------------------------------------------------------------

diff --git a/arch/i386/kernel/setup.c b/arch/i386/kernel/setup.c
index 98e936d..6b65d8f 100644
--- a/arch/i386/kernel/setup.c
+++ b/arch/i386/kernel/setup.c
@@ -1017,6 +1017,7 @@ e820_all_mapped(unsigned long s, unsigned long e, unsigned type)
 	return 0;
 }
 
+#define MAX_PAE36_PFN 1024*1024*16
 /*
  * Find the highest page frame number we have available
  */
@@ -1044,6 +1045,10 @@ void __init find_max_pfn(void)
 			max_pfn = end;
 		memory_present(0, start, end);
 	}
+	if (max_pfn > MAX_PAE36_PFN) {
+		printk("RAM exceeds maximum supported memory for x86, Truncating to 64GB\n");
+		max_pfn = MAX_PAE36_PFN;
+	}
 }
 
 /*