slab allocators: Provide generic description of alignment defines

Provide description for alignment defines.

Acked-by: David Rientjes <[e-mail]>
Signed-off-by: Christoph Lameter <[e-mail]>
Signed-off-by: Pekka Enberg <[e-mail]>

include/linux/slab.h

···
 #define KMALLOC_MAX_SIZE	(1UL << KMALLOC_SHIFT_HIGH)
 #define KMALLOC_MAX_ORDER	(KMALLOC_SHIFT_HIGH - PAGE_SHIFT)
 
+/*
+ * 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
+ * aligned buffers.
+ */
 #ifndef ARCH_SLAB_MINALIGN
 #define ARCH_SLAB_MINALIGN __alignof__(unsigned long long)
 #endif