x86: fix spontaneous reboot with allyesconfig bzImage

recently the 64-bit allyesconfig bzImage kernel started spontaneously
rebooting during early bootup.

after a few fun hours spent with early init debugging, it turns out
that we've got this rather annoying limit on the size of the kernel
image:

      #define KERNEL_TEXT_SIZE  (40*1024*1024)

which limit my vmlinux just happened to pass:

       text           data       bss        dec       hex   filename
   29703744        4222751   8646224c   42572719   2899baf   vmlinux

40 MB is 42572719 bytes, so my vmlinux was just 1.5% above this limit :-/

So it happily crashed right in head_64.S, which - as we all know - is
the most debuggable code in the whole architecture ;-)

So increase the limit to allow an up to 128MB kernel image to be mapped.
(should anyone be that crazy or lazy)

We have a full 4K of pagetable (level2_kernel_pgt) allocated for these
mappings already, so there's no RAM overhead and the limit was rather
pointless and arbitrary.
Signed-off-by: NIngo Molnar <mingo@elte.hu>

arch/x86/kernel/head_64.S

@@ -379,18 +379,24 @@ NEXT_PAGE(level2_ident_pgt)
 	/* Since I easily can, map the first 1G.
 	 * Don't set NX because code runs from these pages.
 	 */
-	PMDS(0x0000000000000000, __PAGE_KERNEL_LARGE_EXEC, PTRS_PER_PMD)
+	PMDS(0, __PAGE_KERNEL_LARGE_EXEC, PTRS_PER_PMD)
 
 NEXT_PAGE(level2_kernel_pgt)
-	/* 40MB kernel mapping. The kernel code cannot be bigger than that.
-	   When you change this change KERNEL_TEXT_SIZE in page.h too. */
-	/* (2^48-(2*1024*1024*1024)-((2^39)*511)-((2^30)*510)) = 0 */
-	PMDS(0x0000000000000000, __PAGE_KERNEL_LARGE_EXEC|_PAGE_GLOBAL, KERNEL_TEXT_SIZE/PMD_SIZE)
-	/* Module mapping starts here */
-	.fill	(PTRS_PER_PMD - (KERNEL_TEXT_SIZE/PMD_SIZE)),8,0
+	/*
+	 * 128 MB kernel mapping. We spend a full page on this pagetable
+	 * anyway.
+	 *
+	 * The kernel code+data+bss must not be bigger than that.
+	 *
+	 * (NOTE: at +128MB starts the module area, see MODULES_VADDR.
+	 *  If you want to increase this then increase MODULES_VADDR
+	 *  too.)
+	 */
+	PMDS(0, __PAGE_KERNEL_LARGE_EXEC|_PAGE_GLOBAL,
+		KERNEL_TEXT_SIZE/PMD_SIZE)
 
 NEXT_PAGE(level2_spare_pgt)
-	.fill   512,8,0
+	.fill   512, 8, 0
 
 #undef PMDS
 #undef NEXT_PAGE

arch/x86/mm/init_64.c

@@ -172,8 +172,9 @@ set_pte_phys(unsigned long vaddr, unsigned long phys, pgprot_t prot)
 }
 
 /*
- * The head.S code sets up the kernel high mapping from:
- * __START_KERNEL_map to __START_KERNEL_map + KERNEL_TEXT_SIZE
+ * The head.S code sets up the kernel high mapping:
+ *
+ *   from __START_KERNEL_map to __START_KERNEL_map + size (== _end-_text)
  *
  * phys_addr holds the negative offset to the kernel, which is added
  * to the compile time generated pmds. This results in invalid pmds up

include/asm-x86/page_64.h

@@ -47,7 +47,11 @@
 #define __PHYSICAL_MASK_SHIFT	46
 #define __VIRTUAL_MASK_SHIFT	48
 
-#define KERNEL_TEXT_SIZE  (40*1024*1024)
+/*
+ * Kernel image size is limited to 128 MB (see level2_kernel_pgt in
+ * arch/x86/kernel/head_64.S), and it is mapped here:
+ */
+#define KERNEL_TEXT_SIZE	(128*1024*1024)
 #define KERNEL_TEXT_START	_AC(0xffffffff80000000, UL)
 
 #ifndef __ASSEMBLY__