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 e621bd18958ef5dbace3129ebe17a0a475e127d9.

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>

2 files changed, 15 insertions, 5 deletions

diff --git a/arch/x86/include/asm/fixmap.h b/arch/x86/include/asm/fixmap.h
index 14f9890eb495..635f03bb4995 100644
--- a/arch/x86/include/asm/fixmap.h
+++ b/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
diff --git a/arch/x86/mm/ioremap.c b/arch/x86/mm/ioremap.c
index c246d259822d..03c75ffd5c2a 100644
--- a/arch/x86/mm/ioremap.c
+++ b/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",