x86/kdump/64: Restrict kdump kernel reservation to <64TB

Restrict kdump to only reserve crashkernel below 64TB.

The reaons is that the kdump may jump from a 5-level paging mode to a
4-level paging mode kernel. If a 4-level paging mode kdump kernel is put
above 64TB, then the kdump kernel cannot start.

The 1st kernel reserves the kdump kernel region during bootup. At that
point it is not known whether the kdump kernel has 5-level or 4-level
paging support.

To support both restrict the kdump kernel reservation to the lower 64TB
address space to ensure that a 4-level paging mode kdump kernel can be
loaded and successfully started.

[ tglx: Massaged changelog ]
Signed-off-by: Baoquan He <bhe@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Acked-by: Dave Young <dyoung@redhat.com>
Cc: bp@alien8.de
Cc: hpa@zytor.com
Link: https://lkml.kernel.org/r/20190524073810.24298-4-bhe@redhat.com

arch/x86/kernel/setup.c

@@ -453,15 +453,24 @@ static void __init memblock_x86_reserve_range_setup_data(void)
 #define CRASH_ALIGN		SZ_16M
 
 /*
- * Keep the crash kernel below this limit.  On 32 bits earlier kernels
- * would limit the kernel to the low 512 MiB due to mapping restrictions.
+ * Keep the crash kernel below this limit.
+ *
+ * On 32 bits earlier kernels would limit the kernel to the low 512 MiB
+ * due to mapping restrictions.
+ *
+ * On 64bit, kdump kernel need be restricted to be under 64TB, which is
+ * the upper limit of system RAM in 4-level paing mode. Since the kdump
+ * jumping could be from 5-level to 4-level, the jumping will fail if
+ * kernel is put above 64TB, and there's no way to detect the paging mode
+ * of the kernel which will be loaded for dumping during the 1st kernel
+ * bootup.
  */
 #ifdef CONFIG_X86_32
 # define CRASH_ADDR_LOW_MAX	SZ_512M
 # define CRASH_ADDR_HIGH_MAX	SZ_512M
 #else
 # define CRASH_ADDR_LOW_MAX	SZ_4G
-# define CRASH_ADDR_HIGH_MAX	MAXMEM
+# define CRASH_ADDR_HIGH_MAX	SZ_64T
 #endif
 
 static int __init reserve_crashkernel_low(void)

include/linux/sizes.h

@@ -47,5 +47,6 @@
 #define SZ_2G				0x80000000
 
 #define SZ_4G				_AC(0x100000000, ULL)
+#define SZ_64T				_AC(0x400000000000, ULL)
 
 #endif /* __LINUX_SIZES_H__ */