xen: limit extra memory to a certain ratio of base

If extra memory is very much larger than the base memory size
then all of the base memory can be filled with structures reserved to
describe the extra memory, leaving no space for anything else.

Even at the maximum ratio there will be little space for anything else,
but this change is intended to at least allow the system to boot rather
than crash mysteriously.
Signed-off-by: NJeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>

arch/x86/xen/setup.c

@@ -37,6 +37,18 @@ extern void xen_syscall32_target(void);
 /* Amount of extra memory space we add to the e820 ranges */
 phys_addr_t xen_extra_mem_start, xen_extra_mem_size;
 
+/* 
+ * The maximum amount of extra memory compared to the base size.  The
+ * main scaling factor is the size of struct page.  At extreme ratios
+ * of base:extra, all the base memory can be filled with page
+ * structures for the extra memory, leaving no space for anything
+ * else.
+ * 
+ * 10x seems like a reasonable balance between scaling flexibility and
+ * leaving a practically usable system.
+ */
+#define EXTRA_MEM_RATIO		(10)
+
 static __init void xen_add_extra_mem(unsigned long pages)
 {
 	u64 size = (u64)pages * PAGE_SIZE;
@@ -134,6 +146,7 @@ char * __init xen_memory_setup(void)
 	int rc;
 	struct xen_memory_map memmap;
 	unsigned long extra_pages = 0;
+	unsigned long extra_limit;
 	int i;
 
 	max_pfn = min(MAX_DOMAIN_PAGES, max_pfn);
@@ -196,6 +209,25 @@ char * __init xen_memory_setup(void)
 
 	extra_pages += xen_return_unused_memory(xen_start_info->nr_pages, &e820);
 
+	/*
+	 * Clamp the amount of extra memory to a EXTRA_MEM_RATIO
+	 * factor the base size.  On non-highmem systems, the base
+	 * size is the full initial memory allocation; on highmem it
+	 * is limited to the max size of lowmem, so that it doesn't
+	 * get completely filled.
+	 *
+	 * In principle there could be a problem in lowmem systems if
+	 * the initial memory is also very large with respect to
+	 * lowmem, but we won't try to deal with that here.
+	 */
+	extra_limit = min(EXTRA_MEM_RATIO * min(max_pfn, PFN_DOWN(MAXMEM)),
+			  max_pfn + extra_pages);
+
+	if (extra_limit >= max_pfn)
+		extra_pages = extra_limit - max_pfn;
+	else
+		extra_pages = 0;
+
 	xen_add_extra_mem(extra_pages);
 
 	return "Xen";