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

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Date:	Mon, 27 Jan 2014 14:54:47 +0100
From:	Marek Szyprowski <m.szyprowski@...sung.com>
To:	Akinobu Mita <akinobu.mita@...il.com>,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org
Cc:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.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

Hello,

On 2014-01-14 15:13, 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()
> forgot to fill the memory region with zero if it was allocated by
> dma_alloc_from_contiguous()

I just wonder how it will work with high mem? I've didn't check the x86
dma mapping code yet, but page_address() works only for pages, which comes
from low memory. In other patches you have added an option to place CMA
area anywhere in the memory. Is the x86 pci dma code ready for the case
when cma area is put into high mem and direct mappings are not available?

> 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);
>   }

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland

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