SUNRPC: Add svc_rqst_replace_page() API

Replacing a page in rq_pages[] requires a get_page(), which is a
bus-locked operation, and a put_page(), which can be even more
costly.

To reduce the cost of replacing a page in rq_pages[], batch the
put_page() operations by collecting "freed" pages in a pagevec,
and then release those pages when the pagevec is full. This
pagevec is also emptied when each RPC completes.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

include/linux/sunrpc/svc.h

@@ -19,6 +19,7 @@
 #include <linux/sunrpc/svcauth.h>
 #include <linux/wait.h>
 #include <linux/mm.h>
+#include <linux/pagevec.h>
 
 /* statistics for svc_pool structures */
 struct svc_pool_stats {
@@ -256,6 +257,7 @@ struct svc_rqst {
 	struct page *		*rq_next_page; /* next reply page to use */
 	struct page *		*rq_page_end;  /* one past the last page */
 
+	struct pagevec		rq_pvec;
 	struct kvec		rq_vec[RPCSVC_MAXPAGES]; /* generally useful.. */
 	struct bio_vec		rq_bvec[RPCSVC_MAXPAGES];
 
@@ -502,6 +504,8 @@ struct svc_rqst *svc_rqst_alloc(struct svc_serv *serv,
 					struct svc_pool *pool, int node);
 struct svc_rqst *svc_prepare_thread(struct svc_serv *serv,
 					struct svc_pool *pool, int node);
+void		   svc_rqst_replace_page(struct svc_rqst *rqstp,
+					 struct page *page);
 void		   svc_rqst_free(struct svc_rqst *);
 void		   svc_exit_thread(struct svc_rqst *);
 unsigned int	   svc_pool_map_get(void);

net/sunrpc/svc.c

@@ -838,6 +838,27 @@ svc_set_num_threads_sync(struct svc_serv *serv, struct svc_pool *pool, int nrser
 }
 EXPORT_SYMBOL_GPL(svc_set_num_threads_sync);
 
+/**
+ * svc_rqst_replace_page - Replace one page in rq_pages[]
+ * @rqstp: svc_rqst with pages to replace
+ * @page: replacement page
+ *
+ * When replacing a page in rq_pages, batch the release of the
+ * replaced pages to avoid hammering the page allocator.
+ */
+void svc_rqst_replace_page(struct svc_rqst *rqstp, struct page *page)
+{
+	if (*rqstp->rq_next_page) {
+		if (!pagevec_space(&rqstp->rq_pvec))
+			__pagevec_release(&rqstp->rq_pvec);
+		pagevec_add(&rqstp->rq_pvec, *rqstp->rq_next_page);
+	}
+
+	get_page(page);
+	*(rqstp->rq_next_page++) = page;
+}
+EXPORT_SYMBOL_GPL(svc_rqst_replace_page);
+
 /*
  * Called from a server thread as it's exiting. Caller must hold the "service
  * mutex" for the service.

net/sunrpc/svc_xprt.c

@@ -539,6 +539,7 @@ static void svc_xprt_release(struct svc_rqst *rqstp)
 	kfree(rqstp->rq_deferred);
 	rqstp->rq_deferred = NULL;
 
+	pagevec_release(&rqstp->rq_pvec);
 	svc_free_res_pages(rqstp);
 	rqstp->rq_res.page_len = 0;
 	rqstp->rq_res.page_base = 0;
@@ -664,6 +665,8 @@ static int svc_alloc_arg(struct svc_rqst *rqstp)
 	struct xdr_buf *arg = &rqstp->rq_arg;
 	unsigned long pages, filled;
 
+	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",