Re: [PATCH v7 07/12] dma-mapping: introduce dma_has_iommu()

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On Mon, Oct 9, 2017 at 12:18 PM, Jason Gunthorpe
<jgunthorpe@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Mon, Oct 09, 2017 at 12:05:30PM -0700, Dan Williams wrote:
>> On Mon, Oct 9, 2017 at 11:58 AM, Jason Gunthorpe
>> <jgunthorpe@xxxxxxxxxxxxxxxxxxxx> wrote:
>> > On Fri, Oct 06, 2017 at 03:35:54PM -0700, Dan Williams wrote:
>> >> otherwise be quiesced. The need for this knowledge is driven by a need
>> >> to make RDMA transfers to DAX mappings safe. If the DAX file's block map
>> >> changes we need to be to reliably stop accesses to blocks that have been
>> >> freed or re-assigned to a new file.
>> >
>> > If RDMA is driving this need, why not invalidate backing RDMA MRs
>> > instead of requiring a IOMMU to do it? RDMA MR are finer grained and
>> > do not suffer from the re-use problem David W. brought up with IOVAs..
>>
>> Sounds promising. All I want in the end is to be sure that the kernel
>> is enabled to stop any in-flight RDMA at will without asking
>> userspace. Does this require per-RDMA driver opt-in or is there a
>> common call that can be made?
>
> I don't think this has ever come up in the context of an all-device MR
> invalidate requirement. Drivers already have code to invalidate
> specifc MRs, but to find all MRs that touch certain pages and then
> invalidate them would be new code.
>
> We also have ODP aware drivers that can retarget a MR to new
> physical pages. If the block map changes DAX should synchronously
> retarget the ODP MR, not halt DMA.

Have a look at the patch [1], I don't touch the ODP path.

> Most likely ODP & DAX would need to be used together to get robust
> user applications, as having the user QP's go to an error state at
> random times (due to DMA failures) during operation is never going to
> be acceptable...

It's not random. The process that set up the mapping and registered
the memory gets SIGIO when someone else tries to modify the file map.
That process then gets /proc/sys/fs/lease-break-time seconds to fix
the problem before the kernel force revokes the DMA access.

It's otherwise not acceptable to allow DMA into random locations when
the file map changes.

> Perhaps you might want to initially only support ODP MR mappings with
> DAX and then the DMA fencing issue goes away?

I'd rather try to fix the non-ODP DAX case instead of just turning it off.

[1]: https://patchwork.kernel.org/patch/9991681/
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