Re: block: DMA alignment of IO buffer allocated from slab

[Bart Van Assche <bvanassche@acm.org>]

On Mon, 2018-09-24 at 11:57 -0700, Matthew Wilcox wrote:
> On Mon, Sep 24, 2018 at 09:19:44AM -0700, Bart Van Assche wrote:
> > That means that two buffers allocated with kmalloc() may share a cache line on
> > x86-64. Since it is allowed to use a buffer allocated by kmalloc() for DMA, can
> > this lead to data corruption, e.g. if the CPU writes into one buffer allocated
> > with kmalloc() and a device performs a DMA write to another kmalloc() buffer and
> > both write operations affect the same cache line?
> 
> You're not supposed to use kmalloc memory for DMA.  This is why we have
> dma_alloc_coherent() and friends.

Are you claiming that all drivers that use DMA should use coherent DMA only? If
coherent DMA is the only DMA style that should be used, why do the following
function pointers exist in struct dma_map_ops?

	void (*sync_single_for_cpu)(struct device *dev,
				    dma_addr_t dma_handle, size_t size,
				    enum dma_data_direction dir);
	void (*sync_single_for_device)(struct device *dev,
				       dma_addr_t dma_handle, size_t size,
				       enum dma_data_direction dir);
	void (*sync_sg_for_cpu)(struct device *dev,
				struct scatterlist *sg, int nents,
				enum dma_data_direction dir);
	void (*sync_sg_for_device)(struct device *dev,
				   struct scatterlist *sg, int nents,
				   enum dma_data_direction dir);

Thanks,

Bart.