um: Centralize stub size calculations

Currently, the stub size is calculated in multiple places. Define
a macro that performs the calculation so that the code is easier
to read and maintain.

Signed-off-by: Tiwei Bie <[e-mail]>
Signed-off-by: Johannes Berg <[e-mail]>

arch/um/include/shared/as-layout.h

···
 #define STUB_CODE STUB_START
 #define STUB_DATA (STUB_CODE + UM_KERN_PAGE_SIZE)
 #define STUB_DATA_PAGES 2
-#define STUB_END (STUB_DATA + STUB_DATA_PAGES * UM_KERN_PAGE_SIZE)
+#define STUB_SIZE ((1 + STUB_DATA_PAGES) * UM_KERN_PAGE_SIZE)
+#define STUB_END (STUB_START + STUB_SIZE)
 
 #ifndef __ASSEMBLER__
 

arch/um/kernel/um_arch.c

···
 
 	host_task_size = get_top_address(envp);
 	/* reserve a few pages for the stubs */
-	stub_start = host_task_size - STUB_DATA_PAGES * PAGE_SIZE;
-	/* another page for the code portion */
-	stub_start -= PAGE_SIZE;
+	stub_start = host_task_size - STUB_SIZE;
 	host_task_size = stub_start;
 
 	/* Limit TASK_SIZE to what is addressable by the page table */

arch/x86/um/shared/sysdep/stub_32.h

···
 		"subl %0,%%esp ;"					\
 		"movl %1, %%eax ; "					\
 		"call *%%eax ;"						\
-		:: "i" ((1 + STUB_DATA_PAGES) * UM_KERN_PAGE_SIZE),	\
+		:: "i" (STUB_SIZE),					\
 		   "i" (&fn))
 
 static __always_inline void

arch/x86/um/shared/sysdep/stub_64.h

···
 		"subq %0,%%rsp ;"					\
 		"movq %1,%%rax ;"					\
 		"call *%%rax ;"						\
-		:: "i" ((1 + STUB_DATA_PAGES) * UM_KERN_PAGE_SIZE),	\
+		:: "i" (STUB_SIZE),					\
 		   "i" (&fn))
 
 static __always_inline void