xen/balloon: make sure we only include remaining extra ram

If the user specifies mem= on the kernel command line, some or all
of the extra memory E820 region may be clipped away, so make sure
we don't try to add more extra memory than exists in E820.
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>

drivers/xen/balloon.c

@@ -50,6 +50,7 @@
 #include <asm/pgtable.h>
 #include <asm/uaccess.h>
 #include <asm/tlb.h>
+#include <asm/e820.h>
 
 #include <asm/xen/hypervisor.h>
 #include <asm/xen/hypercall.h>
@@ -391,7 +392,7 @@ static struct notifier_block xenstore_notifier;
 
 static int __init balloon_init(void)
 {
-	unsigned long pfn;
+	unsigned long pfn, extra_pfn_end;
 	struct page *page;
 
 	if (!xen_pv_domain())
@@ -412,8 +413,10 @@ static int __init balloon_init(void)
 	register_balloon(&balloon_sysdev);
 
 	/* Initialise the balloon with excess memory space. */
+	extra_pfn_end = min(e820_end_of_ram_pfn(),
+			    (unsigned long)PFN_DOWN(xen_extra_mem_start + xen_extra_mem_size));
 	for (pfn = PFN_UP(xen_extra_mem_start);
-	     pfn < PFN_DOWN(xen_extra_mem_start + xen_extra_mem_size);
+	     pfn < extra_pfn_end;
 	     pfn++) {
 		page = pfn_to_page(pfn);
 		/* totalram_pages doesn't include the boot-time