1 files changed, 32 insertions, 0 deletions

diff --git a/arch/x86/xen/setup.c b/arch/x86/xen/setup.c
index 1e85e26efa69..6c9039e92f81 100644
--- a/arch/x86/xen/setup.c
+++ b/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";