um: Report host OOM more nicely

If UML runs on the host side out of memory, report this
condition more nicely.
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/um/kernel/tlb.c

@@ -50,6 +50,13 @@ struct host_vm_change {
 	   .index	= 0, \
 	   .force	= force })
 
+static void report_enomem(void)
+{
+	printk(KERN_ERR "UML ran out of memory on the host side! "
+			"This can happen due to a memory limitation or "
+			"vm.max_map_count has been reached.\n");
+}
+
 static int do_ops(struct host_vm_change *hvc, int end,
 		  int finished)
 {
@@ -81,6 +88,9 @@ static int do_ops(struct host_vm_change *hvc, int end,
 		}
 	}
 
+	if (ret == -ENOMEM)
+		report_enomem();
+
 	return ret;
 }
 
@@ -433,8 +443,12 @@ void flush_tlb_page(struct vm_area_struct *vma, unsigned long address)
 	else if (pte_newprot(*pte))
 		err = protect(mm_id, address, PAGE_SIZE, prot, 1, &flush);
 
-	if (err)
+	if (err) {
+		if (err == -ENOMEM)
+			report_enomem();
+
 		goto kill;
+	}
 
 	*pte = pte_mkuptodate(*pte);