SUNRPC: Be even lazier about releasing pages

A single RPC transaction that touches only a couple of pages means
rq_pvec will not be even close to full in svc_xpt_release(). This is
a common case.

Instead, just leave the pages in rq_pvec until it is completely
full. This improves the efficiency of the batch release mechanism
on workloads that involve small RPC messages.

The rq_pvec is also fully emptied just before thread exit.
Reviewed-by: Calum Mackay <calum.mackay@oracle.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

net/sunrpc/svc.c

@@ -649,6 +649,8 @@ svc_rqst_alloc(struct svc_serv *serv, struct svc_pool *pool, int node)
 	if (!rqstp)
 		return rqstp;
 
+	pagevec_init(&rqstp->rq_pvec);
+
 	__set_bit(RQ_BUSY, &rqstp->rq_flags);
 	rqstp->rq_server = serv;
 	rqstp->rq_pool = pool;
@@ -894,6 +896,7 @@ void svc_rqst_release_pages(struct svc_rqst *rqstp)
 void
 svc_rqst_free(struct svc_rqst *rqstp)
 {
+	pagevec_release(&rqstp->rq_pvec);
 	svc_release_buffer(rqstp);
 	if (rqstp->rq_scratch_page)
 		put_page(rqstp->rq_scratch_page);

net/sunrpc/svc_xprt.c

@@ -541,7 +541,6 @@ static void svc_xprt_release(struct svc_rqst *rqstp)
 	kfree(rqstp->rq_deferred);
 	rqstp->rq_deferred = NULL;
 
-	pagevec_release(&rqstp->rq_pvec);
 	svc_rqst_release_pages(rqstp);
 	rqstp->rq_res.page_len = 0;
 	rqstp->rq_res.page_base = 0;
@@ -667,8 +666,6 @@ static int svc_alloc_arg(struct svc_rqst *rqstp)
 	struct xdr_buf *arg = &rqstp->rq_arg;
 	unsigned long pages, filled, ret;
 
-	pagevec_init(&rqstp->rq_pvec);
-
 	pages = (serv->sv_max_mesg + 2 * PAGE_SIZE) >> PAGE_SHIFT;
 	if (pages > RPCSVC_MAXPAGES) {
 		pr_warn_once("svc: warning: pages=%lu > RPCSVC_MAXPAGES=%lu\n",