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

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Date:	Fri, 18 Apr 2014 00:40:42 +0900
From:	Akinobu Mita <akinobu.mita@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	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 v3 1/5] x86: make dma_alloc_coherent() return zeroed
 memory if CMA is enabled

2014-04-17 4:44 GMT+09:00 Andrew Morton <akpm@...ux-foundation.org>:
> On Tue, 15 Apr 2014 22:08:45 +0900 Akinobu Mita <akinobu.mita@...il.com> 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()?

You're right.  Clearing __GFP_ZERO bit in gfp flags before allocating by
alloc_pages_node() can fix this duplicated memset.

> 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?

Sounds good.  If it also handles __GFP_WAIT, We can remove __GFP_WAIT
check which is almost always required before calling
dma_alloc_from_contiguous().

>> --- 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;

I'll soon prepare a follow-up patch to clear __GFP_ZERO like

+        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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