Re: [PATCH v3 1/2] xprtrdma: Fix DMA scatter-gather list mapping imbalance

[Chuck Lever <chuck.lever@oracle.com>]

> On Feb 12, 2020, at 2:30 PM, Jason Gunthorpe <jgg@ziepe.ca> wrote:
> 
> On Wed, Feb 12, 2020 at 02:09:03PM -0500, Chuck Lever wrote:
>> 
>> 
>>> On Feb 12, 2020, at 2:05 PM, Jason Gunthorpe <jgg@ziepe.ca> wrote:
>>> 
>>> On Wed, Feb 12, 2020 at 01:38:59PM -0500, Chuck Lever wrote:
>>>> 
>>>>> On Feb 12, 2020, at 1:26 PM, Jason Gunthorpe <jgg@ziepe.ca> wrote:
>>>>> 
>>>>> On Wed, Feb 12, 2020 at 11:12:30AM -0500, Chuck Lever wrote:
>>>>>> The @nents value that was passed to ib_dma_map_sg() has to be passed
>>>>>> to the matching ib_dma_unmap_sg() call. If ib_dma_map_sg() choses to
>>>>>> concatenate sg entries, it will return a different nents value than
>>>>>> it was passed.
>>>>>> 
>>>>>> The bug was exposed by recent changes to the AMD IOMMU driver, which
>>>>>> enabled sg entry concatenation.
>>>>>> 
>>>>>> Looking all the way back to commit 4143f34e01e9 ("xprtrdma: Port to
>>>>>> new memory registration API") and reviewing other kernel ULPs, it's
>>>>>> not clear that the frwr_map() logic was ever correct for this case.
>>>>>> 
>>>>>> Reported-by: Andre Tomt <andre@tomt.net>
>>>>>> Suggested-by: Robin Murphy <robin.murphy@arm.com>
>>>>>> Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
>>>>>> Cc: stable@vger.kernel.org # v5.5
>>>>>> net/sunrpc/xprtrdma/frwr_ops.c |   13 +++++++------
>>>>>> 1 file changed, 7 insertions(+), 6 deletions(-)
>>>>> 
>>>>> Yep
>>>>> 
>>>>> Reviewed-by: Jason Gunthorpe <jgg@mellanox.com>
>>>> 
>>>> Thanks.
>>>> 
>>>> Wondering if it makes sense to add a Fixes tag for the AMD IOMMU commit
>>>> where NFS/RDMA stopped working, rather than the "Cc: stable # v5.5".
>>>> 
>>>> Fixes: be62dbf554c5 ("iommu/amd: Convert AMD iommu driver to the dma-iommu api")
>>> 
>>> Not really, this was broken for other configurations besides AMD
>> 
>> Agreed, but the bug seems to have been inconsequential until now?
> 
> I imagine it would get you on ARM or other archs, IIRC.

That's certainly plausible, but I haven't received explicit bug reports
in this area. (I'm not at all saying that such bugs categorically do
not exist).

In any event, practical matters: the posted patch applies back to v5.4,
but fails to apply starting with v5.3.

I think we can leave the "Cc: stable # v5.5"; and I'm open to requests
to backport this simple fix onto earlier stable kernels (back to v4.4),
which can be handled case-by-case. 'Salright?

--
Chuck Lever