x86: Lift restriction on the location of FIX_BTMAP_*

The early ioremap fixmap entries cover half (or for 32-bit
non-PAE, a quarter) of a page table, yet they got
uncondtitionally aligned so far to a 256-entry boundary. This is
not necessary if the range of page table entries anyway falls
into a single page table.

This buys back, for (theoretically) 50% of all configurations
(25% of all non-PAE ones), at least some of the lowmem
necessarily lost with commit e621bd18.
Signed-off-by: Jan Beulich <jbeulich@novell.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
LKML-Reference: <4B2BB66F0200007800026AD6@vpn.id2.novell.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/include/asm/fixmap.h

@@ -118,14 +118,20 @@ enum fixed_addresses {
 	 * 256 temporary boot-time mappings, used by early_ioremap(),
 	 * before ioremap() is functional.
 	 *
-	 * We round it up to the next 256 pages boundary so that we
-	 * can have a single pgd entry and a single pte table:
+	 * If necessary we round it up to the next 256 pages boundary so
+	 * that we can have a single pgd entry and a single pte table:
 	 */
 #define NR_FIX_BTMAPS		64
 #define FIX_BTMAPS_SLOTS	4
-	FIX_BTMAP_END = __end_of_permanent_fixed_addresses + 256 -
-			(__end_of_permanent_fixed_addresses & 255),
-	FIX_BTMAP_BEGIN = FIX_BTMAP_END + NR_FIX_BTMAPS*FIX_BTMAPS_SLOTS - 1,
+#define TOTAL_FIX_BTMAPS	(NR_FIX_BTMAPS * FIX_BTMAPS_SLOTS)
+	FIX_BTMAP_END =
+	 (__end_of_permanent_fixed_addresses ^
+	  (__end_of_permanent_fixed_addresses + TOTAL_FIX_BTMAPS - 1)) &
+	 -PTRS_PER_PTE
+	 ? __end_of_permanent_fixed_addresses + TOTAL_FIX_BTMAPS -
+	   (__end_of_permanent_fixed_addresses & (TOTAL_FIX_BTMAPS - 1))
+	 : __end_of_permanent_fixed_addresses,
+	FIX_BTMAP_BEGIN = FIX_BTMAP_END + TOTAL_FIX_BTMAPS - 1,
 #ifdef CONFIG_PROVIDE_OHCI1394_DMA_INIT
 	FIX_OHCI1394_BASE,
 #endif

arch/x86/mm/ioremap.c

@@ -422,6 +422,10 @@ void __init early_ioremap_init(void)
 	 * The boot-ioremap range spans multiple pmds, for which
 	 * we are not prepared:
 	 */
+#define __FIXADDR_TOP (-PAGE_SIZE)
+	BUILD_BUG_ON((__fix_to_virt(FIX_BTMAP_BEGIN) >> PMD_SHIFT)
+		     != (__fix_to_virt(FIX_BTMAP_END) >> PMD_SHIFT));
+#undef __FIXADDR_TOP
 	if (pmd != early_ioremap_pmd(fix_to_virt(FIX_BTMAP_END))) {
 		WARN_ON(1);
 		printk(KERN_WARNING "pmd %p != %p\n",