Re: [PATCH RFC 2/3] svcrdma: Use device rdma_read_access_flags

[Sagi Grimberg <sagig-LDSdmyG8hGV8YrgS2mwiifqBs+8SCbDb@public.gmane.org>]

>
> I’d like to see our NFS server use the local DMA lkey where it
> makes sense, to avoid the cost of registering and invalidating
> memory.
>
> I have to agree with Tom that once the device’s s/g limit is
> exceeded, the server has to post an RDMA Read WR every few
> pages, and appears to get expensive for large NFS requests.
>
> The current mechanism of statically choosing either FRMR or
> local DMA depending on the device is probably not adequate.
> Maybe we could post all the Read WRs via a single chain? Or
> stick with FRMR when the amount of data to read is significant.
>
> I’ve also tested Christoph’s patch. The logic currently in
> rdma_read_chunk_lcl does not seem up to the task. Once the
> device’s s/g limit is exceeded, the server starts throwing
> local length exceptions, and the client workload hangs.

This is probably because this code path wasn't reached/tested for
a long time as it's hard to find a device that doesn't support FRWR
for a long time now...
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