allow the boot-time sizing of the highmem pool.

arch/i386/kernel/setup.c

@@ -139,6 +139,9 @@ unsigned int mca_pentium_flag;
 /* For PCI or other memory-mapped resources */
 unsigned long pci_mem_start = 0x10000000;
 
+/* user-defined highmem size */
+static unsigned int highmem_pages = -1;
+
 /*
  * Setup options
  */
@@ -640,6 +643,13 @@ static void __init parse_mem_cmdline (char ** cmdline_p)
 		/* acpismp=force forces parsing and use of the ACPI SMP table */
 		if (c == ' ' && !memcmp(from, "acpismp=force", 13))
 			 enable_acpi_smp_table = 1;
+		/*
+		 * highmem=size forces highmem to be exactly 'size' bytes.
+		 * This works even on boxes that have no highmem otherwise.
+		 * This also works to reduce highmem size on bigger boxes.
+		 */
+		if (c == ' ' && !memcmp(from, "highmem=", 8))
+			highmem_pages = memparse(from+8, &from) >> PAGE_SHIFT;
 	
 		c = *(from++);
 		if (!c)
@@ -737,6 +747,14 @@ void __init setup_arch(char **cmdline_p)
 	 */
 	max_low_pfn = max_pfn;
 	if (max_low_pfn > MAXMEM_PFN) {
+		if (highmem_pages == -1)
+			highmem_pages = max_pfn - MAXMEM_PFN;
+		if (highmem_pages + MAXMEM_PFN < max_pfn)
+			max_pfn = MAXMEM_PFN + highmem_pages;
+		if (highmem_pages + MAXMEM_PFN > max_pfn) {
+			printk("only %luMB highmem pages available, ignoring highmem size of %uMB.\n", pages_to_mb(max_pfn - MAXMEM_PFN), pages_to_mb(highmem_pages));
+			highmem_pages = 0;
+		}
 		max_low_pfn = MAXMEM_PFN;
 #ifndef CONFIG_HIGHMEM
 		/* Maximum memory usable is what is directly addressable */
@@ -756,16 +774,37 @@ void __init setup_arch(char **cmdline_p)
 		}
 #endif /* !CONFIG_X86_PAE */
 #endif /* !CONFIG_HIGHMEM */
+	} else {
+		if (highmem_pages == -1)
+			highmem_pages = 0;
+#if CONFIG_HIGHMEM
+		if (highmem_pages >= max_pfn) {
+			printk(KERN_ERR "highmem size specified (%uMB) is bigger than pages available (%luMB)!.\n", pages_to_mb(highmem_pages), pages_to_mb(max_pfn));
+			highmem_pages = 0;
+		}
+		if (highmem_pages) {
+			if (max_low_pfn-highmem_pages < 64*1024*1024/PAGE_SIZE){
+				printk(KERN_ERR "highmem size %uMB results in smaller than 64MB lowmem, ignoring it.\n", pages_to_mb(highmem_pages));
+				highmem_pages = 0;
+			}
+			max_low_pfn -= highmem_pages;
+		}
+#else
+		if (highmem_pages)
+			printk(KERN_ERR "ignoring highmem size on non-highmem kernel!\n");
+#endif
 	}
 
 #ifdef CONFIG_HIGHMEM
 	highstart_pfn = highend_pfn = max_pfn;
-	if (max_pfn > MAXMEM_PFN) {
-		highstart_pfn = MAXMEM_PFN;
+	if (max_pfn > max_low_pfn) {
+		highstart_pfn = max_low_pfn;
+	}
 	printk(KERN_NOTICE "%ldMB HIGHMEM available.\n",
 		pages_to_mb(highend_pfn - highstart_pfn));
-	}
 #endif
+	printk(KERN_NOTICE "%ldMB LOWMEM available.\n",
+			pages_to_mb(max_low_pfn));
 	/*
 	 * Initialize the boot-time allocator (with low memory only):
 	 */