Re: ib_umem_get and DMA_API_DEBUG question

[Gal Pressman <galpress@amazon.com>]

On 27/08/2019 16:17, Jason Gunthorpe wrote:
> On Tue, Aug 27, 2019 at 03:53:29PM +0300, Gal Pressman wrote:
>> On 27/08/2019 15:00, Jason Gunthorpe wrote:
>>> On Tue, Aug 27, 2019 at 11:28:20AM +0300, Gal Pressman wrote:
>>>> On 26/08/2019 17:05, 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.
>>>>
>>>> BTW, on rare occasions I'm seeing the boundary check in check_sg_segment [1]
>>>> fail as well. I don't have a stable repro for it though.
>>>>
>>>> Is this a known issue as well? The comment there states it might be a bug in the
>>>> DMA API implementation, but I'm not sure.
>>>>
>>>> [1] https://elixir.bootlin.com/linux/v5.3-rc3/source/kernel/dma/debug.c#L1230
>>>
>>> Maybe we are missing a dma_set_seg_boundary ?
>>>
>>> PCI uses low defaults:
>>>
>>> 	dma_set_max_seg_size(&dev->dev, 65536);
>>> 	dma_set_seg_boundary(&dev->dev, 0xffffffff);
>>
>> What would you set it to?
> 
> Full 64 bits.
> 
> For umem the driver is responsible to chop up the SGL as required, not
> the core code.

But wouldn't this possibly hide driver bugs? Perhaps even in other flows?