Re: [PATCH v3 1/5] x86: make dma_alloc_coherent() return zeroed memory if CMA is enabled

[Andrew Morton]

On Tue, 15 Apr 2014 22:08:45 +0900 Akinobu Mita <akinobu.mita@xxxxxxxxx> wrote:

> Calling dma_alloc_coherent() with __GFP_ZERO must return zeroed memory.
> 
> But when the contiguous memory allocator (CMA) is enabled on x86 and
> the memory region is allocated by dma_alloc_from_contiguous(), it
> doesn't return zeroed memory.  Because dma_generic_alloc_coherent()
> forgot to fill the memory region with zero if it was allocated by
> dma_alloc_from_contiguous()
> 
> Most implementations of dma_alloc_coherent() return zeroed memory
> regardless of whether __GFP_ZERO is specified.  So this fixes it by
> unconditionally zeroing the allocated memory region.
> 
> Alternatively, we could fix dma_alloc_from_contiguous() to return
> zeroed out memory and remove memset() from all caller of it.  But we
> can't simply remove the memset on arm because __dma_clear_buffer() is
> used there for ensuring cache flushing and it is used in many places.
> Of course we can do redundant memset in dma_alloc_from_contiguous(),
> but I think this patch is less impact for fixing this problem.

But this patch does a duplicated memset if the page was allocated by
alloc_pages_node()?

Would it not be better to pass the gfp_t to dma_alloc_from_contiguous()
and have it implement __GFP_ZERO?  That will fix thsi inefficiency,
will be symmetrical with the other underlying allocators and should
permit the appropriate fixups in arm?


> --- a/arch/x86/kernel/pci-dma.c
> +++ b/arch/x86/kernel/pci-dma.c
> @@ -97,7 +97,6 @@ void *dma_generic_alloc_coherent(struct device *dev, size_t size,
>  
>  	dma_mask = dma_alloc_coherent_mask(dev, flag);
>  
> -	flag |= __GFP_ZERO;
>  again:
>  	page = NULL;
>  	/* CMA can be used only in the context which permits sleeping */
> @@ -120,7 +119,7 @@ again:
>  
>  		return NULL;
>  	}
> -
> +	memset(page_address(page), 0, size);
>  	*dma_addr = addr;
>  	return page_address(page);
>  }
> -- 
> 1.8.3.2
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