Re: [PATCH v2 1/6] xprtrdma: Remove FMR support in rpcrdma_convert_iovs()

[Tom Talpey <tom@talpey.com>]

On 2/3/2021 11:23 AM, Chuck Lever wrote:
> Support for FMR was removed by commit ba69cd122ece ("xprtrdma:
> Remove support for FMR memory registration") [Dec 2018]. That means
> the buffer-splitting behavior of rpcrdma_convert_kvec(), added by
> commit 821c791a0bde ("xprtrdma: Segment head and tail XDR buffers
> on page boundaries") [Mar 2016], is no longer necessary. FRWR
> memory registration handles this case with aplomb.
> 
> Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
> ---
>   net/sunrpc/xprtrdma/rpc_rdma.c |   19 ++++---------------
>   1 file changed, 4 insertions(+), 15 deletions(-)
> 
> diff --git a/net/sunrpc/xprtrdma/rpc_rdma.c b/net/sunrpc/xprtrdma/rpc_rdma.c
> index 8f5d0cb68360..832765f3ebba 100644
> --- a/net/sunrpc/xprtrdma/rpc_rdma.c
> +++ b/net/sunrpc/xprtrdma/rpc_rdma.c
> @@ -204,9 +204,7 @@ rpcrdma_alloc_sparse_pages(struct xdr_buf *buf)
>   	return 0;
>   }
>   
> -/* Split @vec on page boundaries into SGEs. FMR registers pages, not
> - * a byte range. Other modes coalesce these SGEs into a single MR
> - * when they can.
> +/* Convert @vec to a single SGL element.
>    *
>    * Returns pointer to next available SGE, and bumps the total number
>    * of SGEs consumed.
> @@ -215,21 +213,12 @@ static struct rpcrdma_mr_seg *
>   rpcrdma_convert_kvec(struct kvec *vec, struct rpcrdma_mr_seg *seg,
>   		     unsigned int *n)
>   {
> -	u32 remaining, page_offset;
> -	char *base;
> -
> -	base = vec->iov_base;
> -	page_offset = offset_in_page(base);
> -	remaining = vec->iov_len;
> -	while (remaining) {
> +	if (vec->iov_len) {

This is weird. Is it ever possible for a zero-length segment to be
passed? If so, it's obviously wrong to return an uninitialized "seg"
to the caller. I'd suggest simply asserting that iov_len is != 0.

I guess this was an issue in the existing code, but there, it would
only trigger if *all* the segs were zero.

>   		seg->mr_page = NULL;
> -		seg->mr_offset = base;
> -		seg->mr_len = min_t(u32, PAGE_SIZE - page_offset, remaining);
> -		remaining -= seg->mr_len;
> -		base += seg->mr_len;
> +		seg->mr_offset = vec->iov_base;

I thought the previous discussion was that this should be offset_in_page
not the (virtual) iov_base.

Tom.

> +		seg->mr_len = vec->iov_len;
>   		++seg;
>   		++(*n);
> -		page_offset = 0;
>   	}
>   	return seg;
>   }
> 
> 
>