Re: [RFC PATCH 1/2] sunrpc: Replace the rq_bvec array with dynamically-allocated memory

[Jeff Layton <jlayton@kernel.org>]

On Wed, 2025-04-16 at 11:28 -0400, cel@kernel.org wrote:
> From: Chuck Lever <chuck.lever@oracle.com>
> 
> As a step towards making NFSD's maximum rsize and wsize variable,
> replace the fixed-size rq_bvec[] array in struct svc_rqst with a
> chunk of dynamically-allocated memory.
> 
> On a system with 8-byte pointers and 4KB pages, pahole reports that
> the rq_bvec[] array is 4144 bytes. Replacing it with a single
> pointer reduces the size of struct svc_rqst to about 7500 bytes.
> 
> Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
> ---
>  include/linux/sunrpc/svc.h | 2 +-
>  net/sunrpc/svc.c           | 6 ++++++
>  net/sunrpc/svcsock.c       | 7 +++----
>  3 files changed, 10 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/sunrpc/svc.h b/include/linux/sunrpc/svc.h
> index 74658cca0f38..225c385085c3 100644
> --- a/include/linux/sunrpc/svc.h
> +++ b/include/linux/sunrpc/svc.h
> @@ -195,7 +195,7 @@ struct svc_rqst {
>  
>  	struct folio_batch	rq_fbatch;
>  	struct kvec		rq_vec[RPCSVC_MAXPAGES]; /* generally useful.. */
> -	struct bio_vec		rq_bvec[RPCSVC_MAXPAGES];
> +	struct bio_vec		*rq_bvec;

It's a reasonable start.

What would also be good to do here is to replace the invocations of
RPCSVC_MAXPAGES that involve this array with a helper function that
returns the length of it.

For now it could just return RPCSVC_MAXPAGES, but eventually you could
add (e.g.) a rqstp->rq_bvec_len field and use that to indicate how many
entries there are in rq_bvec.

>  
>  	__be32			rq_xid;		/* transmission id */
>  	u32			rq_prog;	/* program number */
> diff --git a/net/sunrpc/svc.c b/net/sunrpc/svc.c
> index e7f9c295d13c..db29819716b8 100644
> --- a/net/sunrpc/svc.c
> +++ b/net/sunrpc/svc.c
> @@ -673,6 +673,7 @@ static void
>  svc_rqst_free(struct svc_rqst *rqstp)
>  {
>  	folio_batch_release(&rqstp->rq_fbatch);
> +	kfree(rqstp->rq_bvec);
>  	svc_release_buffer(rqstp);
>  	if (rqstp->rq_scratch_page)
>  		put_page(rqstp->rq_scratch_page);
> @@ -711,6 +712,11 @@ svc_prepare_thread(struct svc_serv *serv, struct svc_pool *pool, int node)
>  	if (!svc_init_buffer(rqstp, serv->sv_max_mesg, node))
>  		goto out_enomem;
>  
> +	rqstp->rq_bvec = kcalloc_node(RPCSVC_MAXPAGES, sizeof(struct bio_vec),
> +				      GFP_KERNEL, node);
> +	if (!rqstp->rq_bvec)
> +		goto out_enomem;
> +
>  	rqstp->rq_err = -EAGAIN; /* No error yet */
>  
>  	serv->sv_nrthreads += 1;
> diff --git a/net/sunrpc/svcsock.c b/net/sunrpc/svcsock.c
> index 72e5a01df3d3..671640933f18 100644
> --- a/net/sunrpc/svcsock.c
> +++ b/net/sunrpc/svcsock.c
> @@ -713,8 +713,7 @@ static int svc_udp_sendto(struct svc_rqst *rqstp)
>  	if (svc_xprt_is_dead(xprt))
>  		goto out_notconn;
>  
> -	count = xdr_buf_to_bvec(rqstp->rq_bvec,
> -				ARRAY_SIZE(rqstp->rq_bvec), xdr);
> +	count = xdr_buf_to_bvec(rqstp->rq_bvec, RPCSVC_MAXPAGES, xdr);
>  
>  	iov_iter_bvec(&msg.msg_iter, ITER_SOURCE, rqstp->rq_bvec,
>  		      count, rqstp->rq_res.len);
> @@ -1219,8 +1218,8 @@ static int svc_tcp_sendmsg(struct svc_sock *svsk, struct svc_rqst *rqstp,
>  	memcpy(buf, &marker, sizeof(marker));
>  	bvec_set_virt(rqstp->rq_bvec, buf, sizeof(marker));
>  
> -	count = xdr_buf_to_bvec(rqstp->rq_bvec + 1,
> -				ARRAY_SIZE(rqstp->rq_bvec) - 1, &rqstp->rq_res);
> +	count = xdr_buf_to_bvec(rqstp->rq_bvec + 1, RPCSVC_MAXPAGES,
> +				&rqstp->rq_res);
>  
>  	iov_iter_bvec(&msg.msg_iter, ITER_SOURCE, rqstp->rq_bvec,
>  		      1 + count, sizeof(marker) + rqstp->rq_res.len);

-- 
Jeff Layton <jlayton@kernel.org>