Cliff Wickman <cpw at sgi.com> writes: > Hi Eric, and all, > > On Mon, Sep 24, 2012 at 08:11:12PM -0700, Eric W. Biederman wrote: >> Cliff Wickman <cpw at sgi.com> writes: >> >> > Gentlemen, >> > >> > In dumping very large memories we are running up against the 896MB >> > limit in SLES11SP2 (3.0.38 kernel). >> >> Odd. That limit should be the maximum address in memory to load the >> crash kernel. Tha limit should have nothing to do with the dump process >> itself. >> >> Are you saying you need more that 512MiB reserved for the crash kernel >> to be able to dump all of the memory in your system? >> >> Eric > > As I noted to Eric privately, yes we need to bump up to crashkernel=1G > or more for some very large memories. > > As an experiment I bumped > +++ linux/arch/x86/kernel/setup.c > @@ -528,7 +528,7 @@ static inline unsigned long long get_tot > #ifdef CONFIG_X86_32 > # define CRASH_KERNEL_ADDR_MAX (512 << 20) > #else > -# define CRASH_KERNEL_ADDR_MAX (896 << 20) > +# define CRASH_KERNEL_ADDR_MAX (1700 << 20) > > And that seems to work. i.e. I'm currently dumping a system where > crashkernel=1G and it seems to be working. > > Am I just living dangerously? So fundamentally this should work. However there have been a lot of kinks and silly limitations in the x86 boot protocol. So it used to be that the bootloader protocol variable ramdisk_max was set to 896M for 32bit kernels. Because the ramdisk could not be located in high memory. Looking today it appears that ramdisk_max has been upped to 4G. I will let you look through the /sbin/kexec source code. As for testing I would up the limit to 4G on x86_64 and see how far you get. The practical question does the system still work with crashkernel=32M when you have raised the limit much higher. So I would test with crashkernel=1G at 2G and see if that works. If that works I figure that in practice all of the bugs are historical and we can forget them. But a sweep through the /sbin/kexec code for the magic number 896 might not be out of order. Eric
