[PATCH] kdump: add default crashkernel reserve kernel config options

[Dave Young <dyoung@redhat.com>]

This is a rework of the crashkernel=auto patches back to 2009 although
I'm not sure if below is the last version of the old effort:
https://lkml.org/lkml/2009/8/12/61
https://lwn.net/Articles/345344/

I changed the original design, instead of adding the auto reserve logic
in code, in this patch just introduce two kernel config options for
the default crashkernel value in MB and the threshold of system memory
in MB so that only reserve default when system memory is equal or
above the threshold.

With the kernel configs distributions can easily change the default
values so that people do not need to manually set kernel cmdline
for common use cases and one can still overwrite the default value
with manual setup or disable it by using crashkernel=0

Signed-off-by: Dave Young <dyoung@redhat.com>
---
Another difference is with original design the crashkernel size scales
with system memory, according to test, large machine may need more
memory in kdump kernel because of several factors:
1. cpu numbers, because of the percpu memory allocated for cpus.
   (kdump can use nr_cpus=1 to workaround this, but some
    arches do not support nr_cpus=X for example powerpc) 
2. IO devices, large system can have a lot of io devices, although we
   can try to only add those device drivers we needed, it is still a
   problem because of some built-in drivers, some stacked logical devices
   eg. device mapper devices, acpi etc.  Even if only considering the
   meta data for driver model it will still be a big number eg. sysfs
   files etc.
3. The minimum memory requirement for some device drivers are big, even
   if some of them have implemented low meory profile.  It is usual to see
   10M memory use for a storage driver.
4. user space initramfs size growing.  Busybox is not usable if we need
   to add udev support and some complicate storage support.  Use dracut
   with systemd, especially networking stuff need more memory.

So probably add another kernel config option to scale the memory size
eg.  CRASHKERNEL_DEFAULT_SCALE_RATIO is also good to have,  in RHEL we
use base_value + system_mem >> (2^14) for x86.  I'm still hesatating
how to describe and add this option. Any suggestions will be appreciated.

 arch/Kconfig        |   16 ++++++++++++++++
 kernel/crash_core.c |   23 ++++++++++++++++++++++-
 2 files changed, 38 insertions(+), 1 deletion(-)

--- linux-x86.orig/arch/Kconfig
+++ linux-x86/arch/Kconfig
@@ -10,6 +10,22 @@ config KEXEC_CORE
 	select CRASH_CORE
 	bool
 
+config CRASHKERNEL_DEFAULT_THRESHOLD_MB
+	int "System memory size threshold for kdump memory default reserving"
+	depends on CRASH_CORE
+	default 0
+	help
+	  CRASHKERNEL_DEFAULT_MB is used as default crashkernel value if
+	  the system memory size is equal or bigger than the threshold.
+
+config CRASHKERNEL_DEFAULT_MB
+	int "Default crashkernel memory size reserved for kdump"
+	depends on CRASH_CORE
+	default 0
+	help
+	  This is used as the default kdump reserved memory size in MB.
+	  crashkernel=X kernel cmdline can overwrite this value.
+
 config HAVE_IMA_KEXEC
 	bool
 
--- linux-x86.orig/kernel/crash_core.c
+++ linux-x86/kernel/crash_core.c
@@ -9,6 +9,7 @@
 #include <linux/crash_core.h>
 #include <linux/utsname.h>
 #include <linux/vmalloc.h>
+#include <linux/sizes.h>
 
 #include <asm/page.h>
 #include <asm/sections.h>
@@ -143,6 +144,24 @@ static int __init parse_crashkernel_simp
 	return 0;
 }
 
+static int __init get_crashkernel_default(unsigned long long system_ram,
+					  unsigned long long *size)
+{
+	unsigned long long sz = CONFIG_CRASHKERNEL_DEFAULT_MB;
+	unsigned long long thres = CONFIG_CRASHKERNEL_DEFAULT_THRESHOLD_MB;
+
+	thres *= SZ_1M;
+	sz *= SZ_1M;
+
+	if (sz >= system_ram || system_ram < thres) {
+		pr_debug("crashkernel default size can not be used.\n");
+		return -EINVAL;
+	}
+	*size = sz;
+
+	return 0;
+}
+
 #define SUFFIX_HIGH 0
 #define SUFFIX_LOW  1
 #define SUFFIX_NULL 2
@@ -240,8 +259,10 @@ static int __init __parse_crashkernel(ch
 	*crash_size = 0;
 	*crash_base = 0;
 
-	ck_cmdline = get_last_crashkernel(cmdline, name, suffix);
+	if (!strstr(cmdline, "crashkernel="))
+		return get_crashkernel_default(system_ram, crash_size);
 
+	ck_cmdline = get_last_crashkernel(cmdline, name, suffix);
 	if (!ck_cmdline)
 		return -EINVAL;