x86: relax RAM check in ioremap()

Kevin Winchester reported the loss of direct rendering, due to:

[    0.588184] agpgart: Detected AGP bridge 0
[    0.588184] agpgart: unable to get memory for graphics translation table.
[    0.588184] agpgart: agp_backend_initialize() failed.
[    0.588207] agpgart-amd64: probe of 0000:00:00.0 failed with error -12

and bisected it down to:

  commit 266b9f8727976769e2ed2dad77ac9295f37e321e
  Author: Thomas Gleixner <tglx@linutronix.de>
  Date:   Wed Jan 30 13:34:06 2008 +0100

      x86: fix ioremap RAM check

this check was too strict and caused an ioremap() failure.

the problem is due to the somewhat unclean way of how the GART code
reserves a memory range for its aperture, and how it utilizes it
later on.

Allow RAM pages to be ioremap()-ed too, as long as they are reserved.

Bisected-by: Kevin Winchester <kjwinchester@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Tested-by: Kevin Winchester <kjwinchester@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/mm/ioremap.c

@@ -116,7 +116,7 @@ static void __iomem *__ioremap(unsigned long phys_addr, unsigned long size,
 {
 	void __iomem *addr;
 	struct vm_struct *area;
-	unsigned long offset, last_addr;
+	unsigned long pfn, offset, last_addr;
 	pgprot_t prot;
 
 	/* Don't allow wraparound or zero size */
@@ -133,9 +133,10 @@ static void __iomem *__ioremap(unsigned long phys_addr, unsigned long size,
 	/*
 	 * Don't allow anybody to remap normal RAM that we're using..
 	 */
-	for (offset = phys_addr >> PAGE_SHIFT; offset < max_pfn_mapped &&
-	     (offset << PAGE_SHIFT) < last_addr; offset++) {
-		if (page_is_ram(offset))
+	for (pfn = phys_addr >> PAGE_SHIFT; pfn < max_pfn_mapped &&
+	     (pfn << PAGE_SHIFT) < last_addr; pfn++) {
+		if (page_is_ram(pfn) && pfn_valid(pfn) &&
+		    !PageReserved(pfn_to_page(pfn)))
 			return NULL;
 	}