Re: [RFC PATCH v1 4/8] xprtrdma: return -EREMOTEIO on P2PDMA map failure

[Leon Romanovsky <leon@kernel.org>]

On Tue, Oct 06, 2026 at 11:32:44PM +0000, Pranjal Shrivastava wrote:
> ib_dma_map_sg() returns 0 on any failure, so frwr_map() cannot tell
> why a mapping failed and reports -EIO. Use ib_dma_map_sgtable_attrs()
> instead, which returns the reason. Pass up -EREMOTEIO, which means
> the device cannot reach the P2PDMA memory, so the RPC fails with the
> same error as other P2PDMA refusals. Every other mapping failure
> still returns -EIO.
> 
> Signed-off-by: Pranjal Shrivastava <praan@google.com>
> ---
>  net/sunrpc/xprtrdma/frwr_ops.c | 15 ++++++++++-----
>  1 file changed, 10 insertions(+), 5 deletions(-)
> 
> diff --git a/net/sunrpc/xprtrdma/frwr_ops.c b/net/sunrpc/xprtrdma/frwr_ops.c
> index e83cef19e656..a55d5e11c0e2 100644
> --- a/net/sunrpc/xprtrdma/frwr_ops.c
> +++ b/net/sunrpc/xprtrdma/frwr_ops.c
> @@ -293,7 +293,8 @@ int frwr_map(struct rpcrdma_xprt *r_xprt,
>  	bool sg_gaps = ep->re_mrtype == IB_MR_TYPE_SG_GAPS;
>  	unsigned int max_depth = ep->re_max_fr_depth;
>  	struct ib_reg_wr *reg_wr;
> -	int i, n, dma_nents;
> +	struct sg_table sgt;
> +	int i, n, dma_nents, ret;
>  	struct ib_mr *ibmr;
>  	u8 key;
>  
> @@ -381,10 +382,13 @@ int frwr_map(struct rpcrdma_xprt *r_xprt,
>  	mr->mr_dir = rpcrdma_data_dir(writing);
>  	mr->mr_nents = i;
>  
> -	dma_nents = ib_dma_map_sg(ep->re_id->device, mr->mr_sg, mr->mr_nents,
> -				  mr->mr_dir);
> -	if (!dma_nents)
> +	/* Unlike ib_dma_map_sg(), this reports why a mapping failed */
> +	sgt.sgl = mr->mr_sg;
> +	sgt.orig_nents = mr->mr_nents;
> +	ret = ib_dma_map_sgtable_attrs(ep->re_id->device, &sgt, mr->mr_dir, 0);

Please avoid using this function. We are gradually moving away from SG tables
in the RDMA code.

In general, it is never good thing to built API around try-and-fail
approach. It will require from us to be aware of "specific" usage for
the return value from one subsystem by another.

Thanks