Re: [PATCH v8 13/14] IB/core: use MAP_DIRECT to fix / enable RDMA to DAX mappings

[Dan Williams <dan.j.williams-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>]

On Wed, Oct 11, 2017 at 4:54 AM, Joerg Roedel <joro-zLv9SwRftAIdnm+yROfE0A@public.gmane.org> wrote:
> On Tue, Oct 10, 2017 at 07:50:12AM -0700, Dan Williams wrote:
>> +static void ib_umem_lease_break(void *__umem)
>> +{
>> +     struct ib_umem *umem = umem;
>> +     struct ib_device *idev = umem->context->device;
>> +     struct device *dev = idev->dma_device;
>> +     struct scatterlist *sgl = umem->sg_head.sgl;
>> +
>> +     iommu_unmap(umem->iommu, sg_dma_address(sgl) & PAGE_MASK,
>> +                     iommu_sg_num_pages(dev, sgl, umem->npages));
>> +}
>
> This looks like an invitation to break your code by random iommu-driver
> changes. There is no guarantee that an iommu-backed dma-api
> implemenation will map exactly iommu_sg_num_pages() pages for a given
> sg-list. In other words, you are mixing the use of the IOMMU-API and the
> DMA-API in an incompatible way that only works because you know the
> internals of the iommu-drivers.
>
> I've seen in another patch that your changes strictly require an IOMMU,
> so you what you should do instead is to switch from the DMA-API to the
> IOMMU-API and do the address-space management yourself.
>

Ok, I'll switch over completely to the iommu api for this. It will
also address Robin's concern.