slab: Handle ARCH_DMA_MINALIGN correctly

James Hogan hit boot problems in next-20130204 on Meta:

  META213-Thread0 DSP [LogF] kobject (4fc03980): tried to init an initialized object, something is seriously wrong.
  META213-Thread0 DSP [LogF]
  META213-Thread0 DSP [LogF] Call trace:
  META213-Thread0 DSP [LogF] [<4000888c>] _show_stack+0x68/0x7c
  META213-Thread0 DSP [LogF] [<400088b4>] _dump_stack+0x14/0x28
  META213-Thread0 DSP [LogF] [<40103794>] _kobject_init+0x58/0x9c
  META213-Thread0 DSP [LogF] [<40103810>] _kobject_create+0x38/0x64
  META213-Thread0 DSP [LogF] [<40103eac>] _kobject_create_and_add+0x14/0x8c
  META213-Thread0 DSP [LogF] [<40190ac4>] _mnt_init+0xd8/0x220
  META213-Thread0 DSP [LogF] [<40190508>] _vfs_caches_init+0xb0/0x160
  META213-Thread0 DSP [LogF] [<401851f4>] _start_kernel+0x274/0x340
  META213-Thread0 DSP [LogF] [<40188424>] _metag_start_kernel+0x58/0x6c
  META213-Thread0 DSP [LogF] [<40000044>] __start+0x44/0x48
  META213-Thread0 DSP [LogF]
  META213-Thread0 DSP [LogF] devtmpfs: initialized
  META213-Thread0 DSP [LogF] L2 Cache: Not present
  META213-Thread0 DSP [LogF] BUG: failure at fs/sysfs/dir.c:736/sysfs_read_ns_type()!
  META213-Thread0 DSP [LogF] Kernel panic - not syncing: BUG!
  META213-Thread0 DSP [Thread Exit] Thread has exited - return code = 4294967295

And bisected the problem to commit 95a05b42 ("slab: Common constants for
kmalloc boundaries").

As it turns out, a fixed KMALLOC_SHIFT_LOW does not work for arches with
higher alignment requirements.

Determine KMALLOC_SHIFT_LOW from ARCH_DMA_MINALIGN instead.
Reported-and-tested-by: James Hogan <james.hogan@imgtec.com>
Signed-off-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>

include/linux/slab.h

@@ -133,6 +133,19 @@ void kfree(const void *);
 void kzfree(const void *);
 size_t ksize(const void *);
 
+/*
+ * Some archs want to perform DMA into kmalloc caches and need a guaranteed
+ * alignment larger than the alignment of a 64-bit integer.
+ * Setting ARCH_KMALLOC_MINALIGN in arch headers allows that.
+ */
+#if defined(ARCH_DMA_MINALIGN) && ARCH_DMA_MINALIGN > 8
+#define ARCH_KMALLOC_MINALIGN ARCH_DMA_MINALIGN
+#define KMALLOC_MIN_SIZE ARCH_DMA_MINALIGN
+#define KMALLOC_SHIFT_LOW ilog2(ARCH_DMA_MINALIGN)
+#else
+#define ARCH_KMALLOC_MINALIGN __alignof__(unsigned long long)
+#endif
+
 #ifdef CONFIG_SLOB
 /*
  * Common fields provided in kmem_cache by all slab allocators
@@ -179,7 +192,9 @@ struct kmem_cache {
 #define KMALLOC_SHIFT_HIGH	((MAX_ORDER + PAGE_SHIFT - 1) <= 25 ? \
 				(MAX_ORDER + PAGE_SHIFT - 1) : 25)
 #define KMALLOC_SHIFT_MAX	KMALLOC_SHIFT_HIGH
+#ifndef KMALLOC_SHIFT_LOW
 #define KMALLOC_SHIFT_LOW	5
+#endif
 #else
 /*
  * SLUB allocates up to order 2 pages directly and otherwise
@@ -187,8 +202,10 @@ struct kmem_cache {
  */
 #define KMALLOC_SHIFT_HIGH	(PAGE_SHIFT + 1)
 #define KMALLOC_SHIFT_MAX	(MAX_ORDER + PAGE_SHIFT)
+#ifndef KMALLOC_SHIFT_LOW
 #define KMALLOC_SHIFT_LOW	3
 #endif
+#endif
 
 /* Maximum allocatable size */
 #define KMALLOC_MAX_SIZE	(1UL << KMALLOC_SHIFT_MAX)
@@ -200,9 +217,7 @@ struct kmem_cache {
 /*
  * Kmalloc subsystem.
  */
-#if defined(ARCH_DMA_MINALIGN) && ARCH_DMA_MINALIGN > 8
-#define KMALLOC_MIN_SIZE ARCH_DMA_MINALIGN
-#else
+#ifndef KMALLOC_MIN_SIZE
 #define KMALLOC_MIN_SIZE (1 << KMALLOC_SHIFT_LOW)
 #endif
 
@@ -289,17 +304,6 @@ static __always_inline int kmalloc_size(int n)
 }
 #endif /* !CONFIG_SLOB */
 
-/*
- * Some archs want to perform DMA into kmalloc caches and need a guaranteed
- * alignment larger than the alignment of a 64-bit integer.
- * Setting ARCH_KMALLOC_MINALIGN in arch headers allows that.
- */
-#ifdef ARCH_DMA_MINALIGN
-#define ARCH_KMALLOC_MINALIGN ARCH_DMA_MINALIGN
-#else
-#define ARCH_KMALLOC_MINALIGN __alignof__(unsigned long long)
-#endif
-
 /*
  * Setting ARCH_SLAB_MINALIGN in arch headers allows a different alignment.
  * Intended for arches that get misalignment faults even for 64 bit integer