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Subject: Re: AMD IOMMU stops RDMA NFS from working since kernel 5.5 (bisected)
To:     Robin Murphy <robin.murphy@arm.com>,
        Chuck Lever <chuck.lever@oracle.com>
Cc:     Tom Murphy <tmurphy@arista.com>,
        Linux NFS Mailing List <linux-nfs@vger.kernel.org>,
        Joerg Roedel <jroedel@suse.de>,
        iommu@lists.linux-foundation.org
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From:   Andre Tomt <andre@tomt.net>
Message-ID: <55784ee1-f221-85e2-d811-b0c5821161a5@tomt.net>
Date:   Tue, 11 Feb 2020 18:53:30 +0100
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On 11.02.2020 17:36, Robin Murphy wrote:
> On 11/02/2020 4:03 pm, Chuck Lever wrote:
>> Robin, your explanation makes sense to me. I can post a fix for this 
>> imbalance later today for Andre to try.
> 
> FWIW here's a quick hack which *should* suppress the concatenation 
> behaviour - if it makes Andre's system any happier then that would 
> indeed point towards dma_map_sg() handling being the culprit.
> 
> Robin.

This hack do indeed make things work again.

> ----->8-----
> diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
> index a2e96a5fd9a7..a6b71bad518e 100644
> --- a/drivers/iommu/dma-iommu.c
> +++ b/drivers/iommu/dma-iommu.c
> @@ -779,7 +779,7 @@ static int __finalise_sg(struct device *dev, struct 
> scatterlist *sg, int nents,
>            * - but doesn't fall at a segment boundary
>            * - and wouldn't make the resulting output segment too long
>            */
> -        if (cur_len && !s_iova_off && (dma_addr & seg_mask) &&
> +        if (0 && cur_len && !s_iova_off && (dma_addr & seg_mask) &&
>               (max_len - cur_len >= s_length)) {
>               /* ...then concatenate it with the previous one */
>               cur_len += s_length;
> @@ -799,6 +799,7 @@ static int __finalise_sg(struct device *dev, struct 
> scatterlist *sg, int nents,
>           if (s_length + s_iova_off < s_iova_len)
>               cur_len = 0;
>       }
> +    WARN_ON(count < nents);
>       return count;
>   }
>