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 14:45 -0400, Chuck Lever wrote:
> On 4/16/25 2:42 PM, Jeff Layton wrote:
> > 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.
> 
> rq_vec, rq_pages, and rq_bvec all have the same entry count (plus or
> minus one) so only one new field is necessary. There are a few other
> places that allocate arrays of size RPCSVC_MAXPAGES that will need
> similar treatment.
>
> Stay tuned for v2.
> 

Ok. I think I didn't articulate this well. Let me try again:

If you're looking to break the assumption that the length of these
arrays is RPCSVC_MAXPAGES, then the thing to do is to eliminate the
places where we make that assumption.

In particular, the two places where you're adding new RPCSVC_MAXPAGES
invocations would be better replaced with a helper function that we can
change the return value of later.

> 
> > >  	__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>