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Subject: Re: [PATCH v2] xprtrdma: Fix DMA scatter-gather list mapping
 imbalance
To:     Chuck Lever <chuck.lever@oracle.com>
Cc:     Linux NFS Mailing List <linux-nfs@vger.kernel.org>,
        iommu@lists.linux-foundation.org
References: <158151473332.515306.1111360128438553868.stgit@morisot.1015granger.net>
 <869DC73D-190E-46AB-B8F8-1A394F92AF41@oracle.com>
From:   Andre Tomt <andre@tomt.net>
Message-ID: <1d2693b1-b37f-c611-91c3-55b567be5274@tomt.net>
Date:   Wed, 12 Feb 2020 17:03:27 +0100
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On 12.02.2020 14:48, Chuck Lever wrote:
>> On Feb 12, 2020, at 8:43 AM, Chuck Lever <chuck.lever@oracle.com> 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 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>
>> ---
>> include/trace/events/rpcrdma.h |    6 ++++--
>> net/sunrpc/xprtrdma/frwr_ops.c |   13 +++++++------
>> 2 files changed, 11 insertions(+), 8 deletions(-)
>>
>> Hi Andre, here's take 2, based on the trace data you sent me.
>> Please let me know if this one fares any better.
>>
>> Changes since v1:
>> - Ensure the correct nents value is passed to ib_map_mr_sg
>> - Record the mr_nents value in the MR trace points
Verified working (with the patch correction) in my environment, with 
some quick testing (mount + some random and bulk I/O)

client, 5.5.3 + patch + amd iommu on = OK
client, 5.5.3 + patch + amd iommu off = OK
client, 5.6-rc1 + patch + amd iommu on = OK

server, 5.5.3 + patch + intel iommu on = OK

Thanks!