Re: ib_umem_get and DMA_API_DEBUG question

[Jason Gunthorpe <jgg@ziepe.ca>]

On Mon, Aug 26, 2019 at 05:05:12PM +0300, Gal Pressman wrote:
> Hi all,
> 
> Lately I've been seeing DMA-API call traces on our automated testing runs which
> complain about overlapping mappings of the same cacheline [1].
> The problem is (most likely) caused due to multiple calls to ibv_reg_mr with the
> same address, which as a result DMA maps the same physical addresses more than 7
> (ACTIVE_CACHELINE_MAX_OVERLAP) times.
> 
> Is this considered a bad behavior by the test? Should this be caught by
> ib_core/driver somehow?
> 
> Thanks,
> Gal
> 
> [1]
> DMA-API: exceeded 7 overlapping mappings of cacheline 0x000000004a0ad6c0
> WARNING: CPU: 56 PID: 63572 at kernel/dma/debug.c:501 add_dma_entry+0x1fd/0x230

I understand it is technically a violation of the DMA Mapping API to
do this, as it can create incoherence in the CPU cache if there are
multiple entities claiming responsibility to flush it around DMA.

So if you see this from a kernel ULP it is probably a bug.

From the userspace flow.. We only support DMA cache coherent archs in
userspace, and there is no way to prevent userspace from registering
the same page multiple times (in fact there are good reasons to do
this), so it is a false message. Would be nice to be able to suppress
it from this path.

Jason