Re: [PATCH] Introduce dmam_zalloc_coherent()

[Marek Szyprowski]

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