Re: [PATCH] kexec: fix 64Gb limit on x86 w/ PAE

[Neil Horman <nhorman@tuxdriver.com>]

Version 2, with Simons consistency fixes and cleanups

Fix up x86 kexec to exclude memory on i686 kernels beyond 64GB limit

We found a problem recently on x86 systems.  If a 32 bit PAE enabled system
contains more then 64GB of physical ram, the kernel will truncate the max_pfn
value to 64GB.  Unfortunately it still leaves all the physical memory regions
present in /proc/iomem.  Since kexec builds its elf headers based on
/proc/iomem the elf headers indicate the size of memory is larger than what the
kernel is willing to address.  The result is that, during a copy of
/proc/vmcore, a read will return -EFAULT when the requested offset is beyond the
64GB range, leaving the seemingly truncated vmcore useless, as the elf headers
indicate memory beyond what the file contains.

The fix for it is pretty straightforward, just ensure that, when on x86 systems,
we don't record any entries in the memory_range array that cross  the 64Gb mark.
This keeps us in line with the kernel and lets the copy finish sucessfully,
providing a workable core

Tested successfully by myself
Originally-authored-by: Dave Anderson <anderson@redhat.com>
Signed-off-by: Neil Horman <nhorman@tuxdriver.com>


 crashdump-x86.c |   15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/kexec/arch/i386/crashdump-x86.c b/kexec/arch/i386/crashdump-x86.c
index 9d37442..9d35b3e 100644
--- a/kexec/arch/i386/crashdump-x86.c
+++ b/kexec/arch/i386/crashdump-x86.c
@@ -34,6 +34,12 @@
 #include "crashdump-x86.h"
 #include <x86/x86-linux.h>
 
+/*
+ * This defines the the last address that we can support access to
+ * with a PAE enabled kernel
+ */
+#define 64G_LIMIT 0xfffffffff
+
 extern struct arch_options_t arch_options;
 
 /* Forward Declaration. */
@@ -114,6 +120,15 @@ static int get_crash_memory_ranges(struct memory_range **range, int *ranges,
 		if (end <= 0x0009ffff)
 			continue;
 
+		/*
+		 *  Exclude any segments starting at or beyond 64GB, and
+		 *  restrict any segments from ending at or beyond 64GB.
+		 */
+		if (start > 64GB_LIMIT)
+			continue;
+		if (end > 64GB_LIMIT)
+			end = 64GB_LIMIT;
+
 		crash_memory_range[memory_ranges].start = start;
 		crash_memory_range[memory_ranges].end = end;
 		crash_memory_range[memory_ranges].type = type;

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