Re: [PATCH v6 01/17] mm/slab: Decouple ARCH_KMALLOC_MINALIGN from ARCH_DMA_MINALIGN

[Vlastimil Babka <vbabka@suse.cz>]

On 5/31/23 17:48, Catalin Marinas wrote:
> In preparation for supporting a kmalloc() minimum alignment smaller than
> the arch DMA alignment, decouple the two definitions. This requires that
> either the kmalloc() caches are aligned to a (run-time) cache-line size
> or the DMA API bounces unaligned kmalloc() allocations. Subsequent
> patches will implement both options.
> 
> After this patch, ARCH_DMA_MINALIGN is expected to be used in static
> alignment annotations and defined by an architecture to be the maximum
> alignment for all supported configurations/SoCs in a single Image.
> Architectures opting in to a smaller ARCH_KMALLOC_MINALIGN will need to
> define its value in the arch headers.
> 
> Since ARCH_DMA_MINALIGN is now always defined, adjust the #ifdef in
> dma_get_cache_alignment() so that there is no change for architectures
> not requiring a minimum DMA alignment.
> 
> Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: Robin Murphy <robin.murphy@arm.com>
> ---
>  include/linux/dma-mapping.h |  2 +-
>  include/linux/slab.h        | 14 +++++++++++---
>  2 files changed, 12 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
> index 0ee20b764000..3288a1339271 100644
> --- a/include/linux/dma-mapping.h
> +++ b/include/linux/dma-mapping.h
> @@ -545,7 +545,7 @@ static inline int dma_set_min_align_mask(struct device *dev,
>  
>  static inline int dma_get_cache_alignment(void)
>  {
> -#ifdef ARCH_DMA_MINALIGN
> +#ifdef ARCH_HAS_DMA_MINALIGN
>  	return ARCH_DMA_MINALIGN;
>  #endif
>  	return 1;
> diff --git a/include/linux/slab.h b/include/linux/slab.h
> index 6b3e155b70bf..50dcf9cfbf62 100644
> --- a/include/linux/slab.h
> +++ b/include/linux/slab.h
> @@ -235,12 +235,20 @@ void kmem_dump_obj(void *object);
>   * alignment larger than the alignment of a 64-bit integer.
>   * Setting ARCH_DMA_MINALIGN in arch headers allows that.
>   */
> -#if defined(ARCH_DMA_MINALIGN) && ARCH_DMA_MINALIGN > 8
> +#ifdef ARCH_DMA_MINALIGN
> +#define ARCH_HAS_DMA_MINALIGN
> +#if ARCH_DMA_MINALIGN > 8 && !defined(ARCH_KMALLOC_MINALIGN)
>  #define ARCH_KMALLOC_MINALIGN ARCH_DMA_MINALIGN
> -#define KMALLOC_MIN_SIZE ARCH_DMA_MINALIGN
> -#define KMALLOC_SHIFT_LOW ilog2(ARCH_DMA_MINALIGN)
> +#endif
>  #else
> +#define ARCH_DMA_MINALIGN __alignof__(unsigned long long)
> +#endif

It seems weird to make slab.h responsible for this part, especially for
#define ARCH_HAS_DMA_MINALIGN, which dma-mapping.h consumes. Maybe it would
be difficult to do differently due to some dependency hell, but minimally I
don't see dma-mapping.h including slab.h so the result is
include-order-dependent? Maybe it's included transitively, but then it's
fragile and would be better to do explicitly?

> +
> +#ifndef ARCH_KMALLOC_MINALIGN
>  #define ARCH_KMALLOC_MINALIGN __alignof__(unsigned long long)
> +#elif ARCH_KMALLOC_MINALIGN > 8
> +#define KMALLOC_MIN_SIZE ARCH_KMALLOC_MINALIGN
> +#define KMALLOC_SHIFT_LOW ilog2(KMALLOC_MIN_SIZE)
>  #endif
>  
>  /*
>