linux-kernel - Re: [PATCH v2 1/5] x86: make dma_alloc_coherent() return zeroed memory if CMA is enabled

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Date:	Wed, 15 Jan 2014 15:09:34 -0500
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Akinobu Mita <akinobu.mita@...il.com>
Cc:	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	David Woodhouse <dwmw2@...radead.org>,
	Don Dutile <ddutile@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Andi Kleen <andi@...stfloor.org>,
	x86@...nel.org, iommu@...ts.linux-foundation.org
Subject: Re: [PATCH v2 1/5] x86: make dma_alloc_coherent() return zeroed
 memory if CMA is enabled

On Tue, Jan 14, 2014 at 11:13:46PM +0900, Akinobu Mita 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()

So why not fix it there to return zeroed out memory?

> 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.
> 
> Cc: Marek Szyprowski <m.szyprowski@...sung.com>
> Cc: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
> Cc: David Woodhouse <dwmw2@...radead.org>
> Cc: Don Dutile <ddutile@...hat.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: "H. Peter Anvin" <hpa@...or.com>
> Cc: Andi Kleen <andi@...stfloor.org>
> Cc: x86@...nel.org
> Cc: iommu@...ts.linux-foundation.org
> Signed-off-by: Akinobu Mita <akinobu.mita@...il.com>
> ---
> New patch from this version
> 
>  arch/x86/kernel/pci-dma.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kernel/pci-dma.c b/arch/x86/kernel/pci-dma.c
> index 872079a..9644405 100644
> --- 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;
>  	if (!(flag & GFP_ATOMIC))
> @@ -118,7 +117,7 @@ again:
>  
>  		return NULL;
>  	}
> -
> +	memset(page_address(page), 0, size);
>  	*dma_addr = addr;
>  	return page_address(page);
>  }
> -- 
> 1.8.3.2
> 
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