[PATCH -v2] x86, mm: Probe memory block size for generic x86 64bit

[Yinghai Lu <yinghai@kernel.org>]

Usually if the system support memory remapping to get back memory for mmio
range, we will have 128M ... 2G at the end.

Try to probe that size.

So we can get less entries in /sys/devices/system/memory/

-v2: don't probe it every time when /sys/../block_size_byte is showed...

Signed-off-by: Yinghai Lu <yinghai@kernel.org>

---
 arch/x86/mm/init_64.c |   34 ++++++++++++++++++++++++++++++----
 1 file changed, 30 insertions(+), 4 deletions(-)

Index: linux-2.6/arch/x86/mm/init_64.c
===================================================================
--- linux-2.6.orig/arch/x86/mm/init_64.c
+++ linux-2.6/arch/x86/mm/init_64.c
@@ -890,17 +890,43 @@ const char *arch_vma_name(struct vm_area
 	return NULL;
 }
 
-#ifdef CONFIG_X86_UV
-unsigned long memory_block_size_bytes(void)
+static unsigned long probe_memory_block_size(void)
 {
+	/* start from 2g */
+	unsigned long bz = 1UL<<31;
+
+#ifdef CONFIG_X86_UV
 	if (is_uv_system()) {
 		printk(KERN_INFO "UV: memory block size 2GB\n");
 		return 2UL * 1024 * 1024 * 1024;
 	}
-	return MIN_MEMORY_BLOCK_SIZE;
-}
 #endif
 
+	/* less than 64g installed */
+	if ((max_pfn << PAGE_SHIFT) < (16UL << 32))
+		return MIN_MEMORY_BLOCK_SIZE;
+
+	/* get the tail size */
+	while (bz > MIN_MEMORY_BLOCK_SIZE) {
+		if (!((max_pfn << PAGE_SHIFT) & (bz - 1)))
+			break;
+		bz >>= 1;
+	}
+
+	printk(KERN_DEBUG "memory block size : %ldMB\n", bz >> 20);
+
+	return bz;
+}
+
+static unsigned long memory_block_size_probed;
+unsigned long memory_block_size_bytes(void)
+{
+	if (!memory_block_size_probed)
+		memory_block_size_probed = probe_memory_block_size();
+
+	return memory_block_size_probed;
+}
+
 #ifdef CONFIG_SPARSEMEM_VMEMMAP
 /*
  * Initialise the sparsemem vmemmap using huge-pages at the PMD level.