From: Christoph Lameter <clameter@sgi.com>

We sometimes do comparisons like

xx & (__GFP_x| __GFP_y)) == GFP_y

This becomes a problem if GFP_y is redefined to be zero.

Both __GFP_HIGHMEM (caused by me) and __GFP_DMA32 (was this way before) can
become zero in order to cause a fall back to ZONE_NORMAL.

We also have a GFP_MASK that is applied before indexing into the nodelists
array.  We can use that mask to avoid having both become zero and instead
use the GFP_MASK to clear the respective bits before retrieving the
zonelist.

- Remove #ifdefs that cause __GFP_HIGHMEM or __GFP_DMA32 to become zero.

- GFP_MASK works nicely to cause fall back to ZONE_NORMAL except if
  CONFIG_ZONE_DMA32 and !CONFIG_HIGHMEM are set. In that case the
  fallback to ZONE_NORMAL does not work. So add a special case
  to mmzone.h to define a special mask that switches off the highmem
  bit for this particular configuration.

Signed-off-by: Christoph Lameter <clameter@sgi.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
---

 include/linux/gfp.h    |    9 +--------
 include/linux/mmzone.h |   21 ++++++++++++++++++---
 2 files changed, 19 insertions(+), 11 deletions(-)

diff -puN include/linux/gfp.h~reduce-max_nr_zones-make-zone_highmem-optional-fix include/linux/gfp.h
--- a/include/linux/gfp.h~reduce-max_nr_zones-make-zone_highmem-optional-fix
+++ a/include/linux/gfp.h
@@ -12,17 +12,10 @@ struct vm_area_struct;
  */
 /* Zone modifiers in GFP_ZONEMASK (see linux/mmzone.h - low three bits) */
 #define __GFP_DMA	((__force gfp_t)0x01u)
-
-#ifdef CONFIG_HIGHMEM
 #define __GFP_HIGHMEM	((__force gfp_t)0x02u)
-#else
-#define __GFP_HIGHMEM	((__force gfp_t)0x00)	/* NORMAL is HIGHMEM */
-#endif
 
-#ifndef CONFIG_ZONE_DMA32
+#if !defined(CONFIG_ZONE_DMA32) && BITS_PER_LONG >= 64
 #define __GFP_DMA32	((__force gfp_t)0x01)	/* ZONE_DMA is ZONE_DMA32 */
-#elif BITS_PER_LONG < 64
-#define __GFP_DMA32	((__force gfp_t)0x00)	/* ZONE_NORMAL is ZONE_DMA32 */
 #else
 #define __GFP_DMA32	((__force gfp_t)0x04)	/* Has own ZONE_DMA32 */
 #endif
diff -puN include/linux/mmzone.h~reduce-max_nr_zones-make-zone_highmem-optional-fix include/linux/mmzone.h
--- a/include/linux/mmzone.h~reduce-max_nr_zones-make-zone_highmem-optional-fix
+++ a/include/linux/mmzone.h
@@ -163,18 +163,33 @@ enum zone_type {
  *
  * NOTE! Make sure this matches the zones in <linux/gfp.h>
  */
-#define GFP_ZONETYPES		((GFP_ZONEMASK + 1) / 2 + 1)    /* Loner */
 
 #ifdef CONFIG_ZONE_DMA32
+
+#ifdef CONFIG_HIGHMEM
+#define GFP_ZONETYPES		((GFP_ZONEMASK + 1) / 2 + 1)    /* Loner */
 #define GFP_ZONEMASK		0x07
-#define	ZONES_SHIFT		2	/* ceil(log2(MAX_NR_ZONES)) */
+#define ZONES_SHIFT		2	/* ceil(log2(MAX_NR_ZONES)) */
+#else
+#define GFP_ZONETYPES		((0x07 + 1) / 2 + 1)    /* Loner */
+/* Mask __GFP_HIGHMEM */
+#define GFP_ZONEMASK		0x05
+#define ZONES_SHIFT		2
+#endif
+
 #else
 #ifdef CONFIG_HIGHMEM
+
 #define GFP_ZONEMASK		0x03
-#define	ZONES_SHIFT		2
+#define ZONES_SHIFT		2
+#define GFP_ZONETYPES		3
+
 #else
+
 #define GFP_ZONEMASK		0x01
 #define ZONES_SHIFT		1
+#define GFP_ZONETYPES		2
+
 #endif
 #endif
 
_