images/farmem: add drain_node feature

This allocates/leaks all pages from the numa node before adding far memory in
there.

images/farmem/farmem_main.c

 #include<linux/module.h>
 #include<linux/kernel.h>
 #include <linux/memory_hotplug.h>
+#include <linux/vmalloc.h>
 
 MODULE_LICENSE("GPL");
 MODULE_AUTHOR("Antoine Kaufmann");
@@ -11,10 +12,39 @@ MODULE_VERSION("0.1");
 static unsigned long base_addr = 0;
 static unsigned long size = 0;
 static int nnid = 1;
+static bool drain_node = false;
 
 module_param(base_addr, ulong, 0);
 module_param(size, ulong, 0);
 module_param(nnid, int, 0);
+module_param(drain_node, bool, 0);
+
+/**
+ * drains all available memory from the specified numa node by allocating (and
+ * leaking) it
+ */
+static void do_drain_node(int nid)
+{
+  unsigned long eaten;
+  struct page *p;
+
+  printk(KERN_INFO "draining node %d\n", nid);
+  if (nid == 0) {
+    printk(KERN_ALERT "draining node 0 is probably a bad idea\n");
+  }
+
+  /* start with large chunks and move to smaller ones */
+  eaten = 0;
+  do {
+    p = alloc_pages_node(nid,
+      GFP_KERNEL | __GFP_NOWARN | __GFP_NORETRY | GFP_NOWAIT | __GFP_THISNODE,
+      0);
+    if (p)
+      eaten += PAGE_SIZE;
+  } while (p);
+
+  printk(KERN_INFO "drained %lu bytes\n", eaten);
+}
 
 static int __init farmem_mod_init(void)
 {
@@ -29,6 +59,9 @@ static int __init farmem_mod_init(void)
     panic("invalid numa node spcified");
   }
 
+  if (drain_node)
+    do_drain_node(nnid);
+
   rc = add_memory_driver_managed(nnid, base_addr, size,
       "System RAM (farmem)", MHP_NONE);
   if (rc) {