RE: [PATCH 16/16] dma-mapping: use exact allocation in dma_alloc_contiguous

[David Laight <David.Laight@ACULAB.COM>]

From: Christoph Hellwig
> Sent: 14 June 2019 14:47
> 
> Many architectures (e.g. arm, m68 and sh) have always used exact
> allocation in their dma coherent allocator, which avoids a lot of
> memory waste especially for larger allocations.  Lift this behavior
> into the generic allocator so that dma-direct and the generic IOMMU
> code benefit from this behavior as well.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
>  include/linux/dma-contiguous.h |  8 +++++---
>  kernel/dma/contiguous.c        | 17 +++++++++++------
>  2 files changed, 16 insertions(+), 9 deletions(-)
> 
> diff --git a/include/linux/dma-contiguous.h b/include/linux/dma-contiguous.h
> index c05d4e661489..2e542e314acf 100644
> --- a/include/linux/dma-contiguous.h
> +++ b/include/linux/dma-contiguous.h
> @@ -161,15 +161,17 @@ static inline struct page *dma_alloc_contiguous(struct device *dev, size_t size,
>  		gfp_t gfp)
>  {
>  	int node = dev ? dev_to_node(dev) : NUMA_NO_NODE;
> -	size_t align = get_order(PAGE_ALIGN(size));
> +	void *cpu_addr = alloc_pages_exact_node(node, size, gfp);
> 
> -	return alloc_pages_node(node, gfp, align);
> +	if (!cpu_addr)
> +		return NULL;
> +	return virt_to_page(p);
>  }

Does this still guarantee that requests for 16k will not cross a 16k boundary?
It looks like you are losing the alignment parameter.

There may be drivers and hardware that also require 12k allocates
to not cross 16k boundaries (etc).

	David

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