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

[Chuck Lever <chuck.lever@oracle.com>]

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.


>>  	__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);
> 

-- 
Chuck Lever