Re: [PATCH 01/10] mm/slab: Decouple ARCH_KMALLOC_MINALIGN from ARCH_DMA_MINALIGN

[Hyeonggon Yoo]

On Tue, Apr 05, 2022 at 02:57:49PM +0100, Catalin Marinas wrote:
> In preparation for supporting a dynamic kmalloc() minimum alignment,
> allow architectures to define ARCH_KMALLOC_MINALIGN independently of
> ARCH_DMA_MINALIGN. In addition, always define ARCH_DMA_MINALIGN even if
> an architecture does not override it.
>

[ +Cc slab maintainer/reviewers ]

I get why you want to set minimum alignment of kmalloc() dynamically.
That's because cache line size can be different and we cannot statically
know that, right?

But I don't get why you are trying to decouple ARCH_KMALLOC_MINALIGN
from ARCH_DMA_MINALIGN. kmalloc'ed buffer is always supposed to be DMA-safe.

I'm afraid this series may break some archs/drivers.

in Documentation/dma-api-howto.rst:
> 2) ARCH_DMA_MINALIGN
> 
>   Architectures must ensure that kmalloc'ed buffer is
>   DMA-safe. Drivers and subsystems depend on it. If an architecture
>   isn't fully DMA-coherent (i.e. hardware doesn't ensure that data in
>   the CPU cache is identical to data in main memory),
>   ARCH_DMA_MINALIGN must be set so that the memory allocator
>   makes sure that kmalloc'ed buffer doesn't share a cache line with
>   the others. See arch/arm/include/asm/cache.h as an example.
>
>   Note that ARCH_DMA_MINALIGN is about DMA memory alignment
>   constraints. You don't need to worry about the architecture data
>   alignment constraints (e.g. the alignment constraints about 64-bit
>   objects).

If I'm missing something, please let me know :)

Thanks,
Hyeonggon

> 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.
> ARCH_KMALLOC_MINALIGN, if different, is the minimum alignment guaranteed
> by kmalloc().
> 
> Signed-off-by: Catalin Marinas <catalin.marinas@xxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> ---
>  include/linux/slab.h | 23 ++++++++++++++++++-----
>  1 file changed, 18 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/slab.h b/include/linux/slab.h
> index 373b3ef99f4e..d58211bdeceb 100644
> --- a/include/linux/slab.h
> +++ b/include/linux/slab.h
> @@ -187,17 +187,30 @@ bool kmem_valid_obj(void *object);
>  void kmem_dump_obj(void *object);
>  #endif
>  
> +/*
> + * slob does not support independent control of ARCH_KMALLOC_MINALIGN and
> + * ARCH_DMA_MINALIGN.
> + */
> +#ifdef CONFIG_SLOB
> +#undef ARCH_KMALLOC_MINALIGN
> +#endif
> +
>  /*
>   * 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.
> + * Setting ARCH_DMA_MINALIGN in arch headers allows that.
>   */
> -#if defined(ARCH_DMA_MINALIGN) && ARCH_DMA_MINALIGN > 8
> +#ifndef ARCH_DMA_MINALIGN
> +#define ARCH_DMA_MINALIGN __alignof__(unsigned long long)
> +#elif 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)
> -#else
> +#endif
> +
> +#ifndef ARCH_KMALLOC_MINALIGN
>  #define ARCH_KMALLOC_MINALIGN __alignof__(unsigned long long)
> +#else
> +#define KMALLOC_MIN_SIZE ARCH_KMALLOC_MINALIGN
> +#define KMALLOC_SHIFT_LOW ilog2(KMALLOC_MIN_SIZE)
>  #endif
>  
>  /*
>