x86/kexec/64: Use gbpages for identity mappings if available

Kexec sets up all identity mappings before booting into the new
kernel, and this will cause extra memory consumption for paging
structures which is quite considerable on modern machines with
huge memory sizes.

E.g. on a 32TB machine that is kdumping, it could waste around
128MB (around 4MB/TB) from the reserved memory after kexec sets
all the identity mappings using the current 2MB page.

Add to that the memory needed for the loaded kdump kernel, initramfs,
etc., and it causes a kexec syscall -NOMEM failure.

As a result, we had to enlarge reserved memory via "crashkernel=X"
to work around this problem.

This causes some trouble for distributions that use policies
to evaluate the proper "crashkernel=X" value for users.

So enable gbpages for kexec mappings.
Signed-off-by: Xunlei Pang <xlpang@redhat.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Dave Young <dyoung@redhat.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Eric Biederman <ebiederm@xmission.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Yinghai Lu <yinghai@kernel.org>
Cc: akpm@linux-foundation.org
Cc: kexec@lists.infradead.org
Link: http://lkml.kernel.org/r/1493862171-8799-2-git-send-email-xlpang@redhat.comSigned-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/machine_kexec_64.c

@@ -122,6 +122,10 @@ static int init_pgtable(struct kimage *image, unsigned long start_pgtable)
 
 	level4p = (pgd_t *)__va(start_pgtable);
 	clear_page(level4p);
+
+	if (direct_gbpages)
+		info.direct_gbpages = true;
+
 	for (i = 0; i < nr_pfn_mapped; i++) {
 		mstart = pfn_mapped[i].start << PAGE_SHIFT;
 		mend   = pfn_mapped[i].end << PAGE_SHIFT;