linux-kernel - Re: [PATCH] Remove GFP_DMA and GFP_DMA32 from flags before passing it into kmalloc.

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Message-ID: <CAHkaATSUH=sKYJZ1NEM6-M8nCYPeqsU6v3JMBQbm_R=ykbkUjg@mail.gmail.com>
Date:	Sun, 28 Sep 2014 08:44:15 +0800
From:	Miles MH Chen <orca.chen@...il.com>
To:	Catalin Marinas <catalin.marinas@....com>
Cc:	Will Deacon <Will.Deacon@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Remove GFP_DMA and GFP_DMA32 from flags before passing it
 into kmalloc.

On Sat, Sep 27, 2014 at 11:13 PM, Catalin Marinas
<catalin.marinas@....com> wrote:
> On 27 Sep 2014, at 16:09, Miles MH Chen <orca.chen@...il.com> wrote:
>> Signed-off-by: Min-Hua Chen <orca.chen@...il.com>
>> ---
>>  arch/arm64/mm/dma-mapping.c |    3 ++-
>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/arm64/mm/dma-mapping.c b/arch/arm64/mm/dma-mapping.c
>> index 4164c5a..273cf6d 100644
>> --- a/arch/arm64/mm/dma-mapping.c
>> +++ b/arch/arm64/mm/dma-mapping.c
>> @@ -103,7 +103,8 @@ static void *__dma_alloc_noncoherent(struct device *dev, size_t size,
>>      ptr = __dma_alloc_coherent(dev, size, dma_handle, flags, attrs);
>>      if (!ptr)
>>          goto no_mem;
>> -    map = kmalloc(sizeof(struct page *) << order, flags & ~GFP_DMA);
>> +    map = kmalloc(sizeof(struct page *) << order,
>> +                    flags & ~(GFP_DMA | GFP_DMA32));
>
> Do you have an explanation on why this is needed (and such explanation
> should also be included in the commit log)? We don’t use ZONE_DMA32 on
> arm64 (we did initially but it was for the wrong reasons).

If GFP_DMA32 is passed to dma_alloc_coherent, the flag will go to kmalloc and
trigger a BUG_ON check in slab allocator:

__dma_alloc_noncoherent
kmalloc
new_slab  BUG_ON(flags & GFP_SLAB_BUG_MASK);

It can be avoided this by removing GFP_DMA32 before passing it to kmalloc
or we should block GFP_DMA32 flag earlier in arch/arm64/dma-mapping.c if
GFP_DMA32 is not allowed in arch/arm64/dma-mapping.c.

Thanks,
MH

>
> Thanks,
>
> Catalin
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