Subject: Re: [PATCH] Introduce dmam_zalloc_coherent()
To: Christoph Hellwig <hch@lst.de>, Michael Moese <michael.moese@men.de>
Cc: Robin Murphy <robin.murphy@arm.com>, linux-kernel@vger.kernel.org
From: Marek Szyprowski <m.szyprowski@samsung.com>
Message-id: <6a551a81-748f-e069-8ffd-20ef13e34453@samsung.com>
Date: Thu, 10 Aug 2017 08:58:54 +0200
User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:52.0) Gecko/20100101
        Thunderbird/52.2.1
MIME-version: 1.0
In-reply-to: <20170809133705.GA25491@lst.de>
Content-type: text/plain; charset=utf-8; format=flowed
Content-transfer-encoding: 7bit
Content-language: en-US
CMS-TYPE: 201P
References: <20170808091535.31371-1-michael.moese@men.de>
 <CGME20170809133713epcas3p39562ad3e1f0e54a09c0549d9d3b85cf6@epcas3p3.samsung.com>
 <20170809133705.GA25491@lst.de>
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1348
Lines: 30

Hi Christoph,

On 2017-08-09 15:37, Christoph Hellwig wrote:
> On Tue, Aug 08, 2017 at 11:15:35AM +0200, Michael Moese wrote:
>> All memory allocation functions have a pendant for allocating zeroed
>> memory, but dmam_alloc_coherent does not have such a pendant.
>> However, it is easier to read dmam_zalloc_coherent than passing an extra
>> flag or, even worse, see memset() after the allocation.
>> This patch adds an inline function dmam_zalloc_coherent(), exactly like
>> the implementation of dma_zalloc_coherent().
> I'm a bit worried about the __GFP_ZERO as we have lots of non-kmalloc
> implementations of these.  But on the other hand we already implement
> dma_zalloc_coherent the same way, which means we'd already buggy.
>
> So I plan to apply this for 4.14, but I also plan to spend some time
> to implement all the existin alloc ops to make sure it's going to work
> fine.

Frankly, since introducing dma_mmap_coherent, dma_alloc_coherent already
clears allocated buffers to avoid potential information leak to userspace.
There are even drivers that rely on such behavior, see
http://lists.infradead.org/pipermail/linux-arm-kernel/2015-April/338804.html

Maybe it would make sense to properly document it and then convert
dma_zalloc* to standard dma_alloc_* calls?

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