Re: [PATCH v2 04/10] sunrpc: Replace the rq_vec array with dynamically-allocated memory

[Jeff Layton <jlayton@kernel.org>]

On Sat, 2025-04-19 at 13:28 -0400, cel@kernel.org wrote:
> From: Chuck Lever <chuck.lever@oracle.com>
> 
> As a step towards making NFSD's maximum rsize and wsize variable at
> run-time, replace the fixed-size rq_vec[] array in struct svc_rqst
> with a chunk of dynamically-allocated memory.
> 
> The rq_vec array is sized assuming request processing will need at
> most one kvec per page in a maximum-sized RPC message.
> 
> On a system with 8-byte pointers and 4KB pages, pahole reports that
> the rq_vec[] array is 4144 bytes. Replacing it with a single
> pointer reduces the size of struct svc_rqst to about 5400 bytes.

nit: so I guess the current struct is ~9k or so? If you're going to
post numbers here, they should probably refer to the same thing.

I'm lazy -- don't make me do math.

> 
> Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
> ---
>  fs/nfsd/nfs4proc.c         | 1 -
>  fs/nfsd/vfs.c              | 2 +-
>  include/linux/sunrpc/svc.h | 2 +-
>  net/sunrpc/svc.c           | 8 +++++++-
>  4 files changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/nfsd/nfs4proc.c b/fs/nfsd/nfs4proc.c
> index b397246dae7b..d1be58b557d1 100644
> --- a/fs/nfsd/nfs4proc.c
> +++ b/fs/nfsd/nfs4proc.c
> @@ -1228,7 +1228,6 @@ nfsd4_write(struct svc_rqst *rqstp, struct nfsd4_compound_state *cstate,
>  	write->wr_how_written = write->wr_stable_how;
>  
>  	nvecs = svc_fill_write_vector(rqstp, &write->wr_payload);
> -	WARN_ON_ONCE(nvecs > ARRAY_SIZE(rqstp->rq_vec));
>  
>  	status = nfsd_vfs_write(rqstp, &cstate->current_fh, nf,
>  				write->wr_offset, rqstp->rq_vec, nvecs, &cnt,
> diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
> index 9abdc4b75813..4eaac3aa7e15 100644
> --- a/fs/nfsd/vfs.c
> +++ b/fs/nfsd/vfs.c
> @@ -1094,7 +1094,7 @@ __be32 nfsd_iter_read(struct svc_rqst *rqstp, struct svc_fh *fhp,
>  		++v;
>  		base = 0;
>  	}
> -	WARN_ON_ONCE(v > ARRAY_SIZE(rqstp->rq_vec));
> +	WARN_ON_ONCE(v > rqstp->rq_maxpages);
>  
>  	trace_nfsd_read_vector(rqstp, fhp, offset, *count);
>  	iov_iter_kvec(&iter, ITER_DEST, rqstp->rq_vec, v, *count);
> diff --git a/include/linux/sunrpc/svc.h b/include/linux/sunrpc/svc.h
> index 96ac12dbb04d..72d016772711 100644
> --- a/include/linux/sunrpc/svc.h
> +++ b/include/linux/sunrpc/svc.h
> @@ -207,7 +207,7 @@ struct svc_rqst {
>  	struct page *		*rq_page_end;  /* one past the last page */
>  
>  	struct folio_batch	rq_fbatch;
> -	struct kvec		rq_vec[RPCSVC_MAXPAGES]; /* generally useful.. */
> +	struct kvec		*rq_vec;
>  	struct bio_vec		rq_bvec[RPCSVC_MAXPAGES];
>  
>  	__be32			rq_xid;		/* transmission id */
> diff --git a/net/sunrpc/svc.c b/net/sunrpc/svc.c
> index 682e11c9be36..5808d4b97547 100644
> --- a/net/sunrpc/svc.c
> +++ b/net/sunrpc/svc.c
> @@ -675,6 +675,7 @@ static void
>  svc_rqst_free(struct svc_rqst *rqstp)
>  {
>  	folio_batch_release(&rqstp->rq_fbatch);
> +	kfree(rqstp->rq_vec);
>  	svc_release_buffer(rqstp);
>  	if (rqstp->rq_scratch_page)
>  		put_page(rqstp->rq_scratch_page);
> @@ -713,6 +714,11 @@ svc_prepare_thread(struct svc_serv *serv, struct svc_pool *pool, int node)
>  	if (!svc_init_buffer(rqstp, serv, node))
>  		goto out_enomem;
>  
> +	rqstp->rq_vec = kcalloc_node(rqstp->rq_maxpages, sizeof(struct kvec),
> +				      GFP_KERNEL, node);
> +	if (!rqstp->rq_vec)
> +		goto out_enomem;
> +
>  	rqstp->rq_err = -EAGAIN; /* No error yet */
>  
>  	serv->sv_nrthreads += 1;
> @@ -1750,7 +1756,7 @@ unsigned int svc_fill_write_vector(struct svc_rqst *rqstp,
>  		++pages;
>  	}
>  
> -	WARN_ON_ONCE(i > ARRAY_SIZE(rqstp->rq_vec));
> +	WARN_ON_ONCE(i > rqstp->rq_maxpages);
>  	return i;
>  }
>  EXPORT_SYMBOL_GPL(svc_fill_write_vector);

-- 
Jeff Layton <jlayton@kernel.org>