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

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

> On Feb 12, 2020, at 2:05 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?

Otherwise we should explore backporting farther into the past.


--
Chuck Lever