x86: create a non-zero sized bm_pte only when needed

Impact: kernel image size reduction

Since in most configurations the pmd page needed maps the same range of
virtual addresses which is also mapped by the earlier inserted one for
covering FIX_DBGP_BASE, that page (and its insertion in the page
tables) can be avoided altogether by detecting the condition at compile
time.
Signed-off-by: NJan Beulich <jbeulich@novell.com>
LKML-Reference: <49B91826.76E4.0078.0@novell.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

arch/x86/mm/ioremap.c

@@ -487,7 +487,12 @@ static int __init early_ioremap_debug_setup(char *str)
 early_param("early_ioremap_debug", early_ioremap_debug_setup);
 
 static __initdata int after_paging_init;
-static pte_t bm_pte[PAGE_SIZE/sizeof(pte_t)] __page_aligned_bss;
+#define __FIXADDR_TOP (-PAGE_SIZE)
+static pte_t bm_pte[(__fix_to_virt(FIX_DBGP_BASE)
+		     ^ __fix_to_virt(FIX_BTMAP_BEGIN)) >> PMD_SHIFT
+		    ? PAGE_SIZE / sizeof(pte_t) : 0] __page_aligned_bss;
+#undef __FIXADDR_TOP
+static __initdata pte_t *bm_ptep;
 
 static inline pmd_t * __init early_ioremap_pmd(unsigned long addr)
 {
@@ -502,6 +507,8 @@ static inline pmd_t * __init early_ioremap_pmd(unsigned long addr)
 
 static inline pte_t * __init early_ioremap_pte(unsigned long addr)
 {
+	if (!sizeof(bm_pte))
+		return &bm_ptep[pte_index(addr)];
 	return &bm_pte[pte_index(addr)];
 }
 
@@ -519,8 +526,14 @@ void __init early_ioremap_init(void)
 		slot_virt[i] = fix_to_virt(FIX_BTMAP_BEGIN - NR_FIX_BTMAPS*i);
 
 	pmd = early_ioremap_pmd(fix_to_virt(FIX_BTMAP_BEGIN));
-	memset(bm_pte, 0, sizeof(bm_pte));
-	pmd_populate_kernel(&init_mm, pmd, bm_pte);
+	if (sizeof(bm_pte)) {
+		memset(bm_pte, 0, sizeof(bm_pte));
+		pmd_populate_kernel(&init_mm, pmd, bm_pte);
+	} else {
+		bm_ptep = pte_offset_kernel(pmd, 0);
+		if (early_ioremap_debug)
+			printk(KERN_INFO "bm_ptep=%p\n", bm_ptep);
+	}
 
 	/*
 	 * The boot-ioremap range spans multiple pmds, for which