1 files changed, 35 insertions, 6 deletions

diff --git a/arch/um/kernel/physmem.c b/arch/um/kernel/physmem.c
index 549ecf3f5857..9034fc8056b4 100644
--- a/arch/um/kernel/physmem.c
+++ b/arch/um/kernel/physmem.c
@@ -57,22 +57,51 @@ void map_memory(unsigned long virt, unsigned long phys, unsigned long len,
 
 extern int __syscall_stub_start;
 
+/**
+ * setup_physmem() - Setup physical memory for UML
+ * @start:      Start address of the physical kernel memory,
+ *              i.e start address of the executable image.
+ * @reserve_end:        end address of the physical kernel memory.
+ * @len:        Length of total physical memory that should be mapped/made
+ *              available, in bytes.
+ * @highmem:    Number of highmem bytes that should be mapped/made available.
+ *
+ * Creates an unlinked temporary file of size (len + highmem) and memory maps
+ * it on the last executable image address (uml_reserved).
+ *
+ * The offset is needed as the length of the total physical memory
+ * (len + highmem) includes the size of the memory used be the executable image,
+ * but the mapped-to address is the last address of the executable image
+ * (uml_reserved == end address of executable image).
+ *
+ * The memory mapped memory of the temporary file is used as backing memory
+ * of all user space processes/kernel tasks.
+ */
 void __init setup_physmem(unsigned long start, unsigned long reserve_end,
                           unsigned long len, unsigned long long highmem)
 {
         unsigned long reserve = reserve_end - start;
-        int pfn = PFN_UP(__pa(reserve_end));
-        int delta = (len - reserve) >> PAGE_SHIFT;
-        int err, offset, bootmap_size;
+        unsigned long pfn = PFN_UP(__pa(reserve_end));
+        unsigned long delta = (len - reserve) >> PAGE_SHIFT;
+        unsigned long offset, bootmap_size;
+        long map_size;
+        int err;
+
+        offset = uml_reserved - uml_physmem;
+        map_size = len - offset;
+        if(map_size <= 0) {
+                printf("Too few physical memory! Needed=%d, given=%d\n",
+                       offset, len);
+                exit(1);
+        }
 
         physmem_fd = create_mem_file(len + highmem);
 
-        offset = uml_reserved - uml_physmem;
         err = os_map_memory((void *) uml_reserved, physmem_fd, offset,
-                            len - offset, 1, 1, 1);
+                            map_size, 1, 1, 1);
         if (err < 0) {
                 printf("setup_physmem - mapping %ld bytes of memory at 0x%p "
-                       "failed - errno = %d\n", len - offset,
+                       "failed - errno = %d\n", map_size,
                        (void *) uml_reserved, err);
                 exit(1);
         }