[RFC PATCH 6/6] sunrpc: Optimize rq_respages allocation in svc_alloc_arg

[Chuck Lever <cel@kernel.org>]

From: Chuck Lever <chuck.lever@oracle.com>

svc_alloc_arg() invokes alloc_pages_bulk() with the full
rq_maxpages count (~259 for 1MB messages) for the rq_respages
array, causing a full-array scan despite most slots holding valid
pages.

svc_rqst_release_pages() NULLs only the range
[rq_respages, rq_next_page) after each RPC, so only that range
contains NULL entries. Limit the rq_respages fill in svc_alloc_arg()
to that range instead of scanning the full array.

svc_init_buffer() initializes rq_next_page to span the entire
rq_respages array, so the first svc_alloc_arg() call fills all
slots.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
---
 include/linux/sunrpc/svc.h | 4 ++++
 net/sunrpc/svc.c           | 1 +
 net/sunrpc/svc_xprt.c      | 8 +++++++-
 3 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/include/linux/sunrpc/svc.h b/include/linux/sunrpc/svc.h
index bb2029e396db..35e9b34e2190 100644
--- a/include/linux/sunrpc/svc.h
+++ b/include/linux/sunrpc/svc.h
@@ -150,6 +150,10 @@ extern u32 svc_max_payload(const struct svc_rqst *rqstp);
  * rq_pages[0] and set rq_pages_nfree to the count of entries
  * consumed. svc_alloc_arg() refills only that many entries.
  *
+ * For rq_respages, svc_rqst_release_pages() NULLs entries in
+ * [rq_respages, rq_next_page) after each RPC. svc_alloc_arg()
+ * refills only that range.
+ *
  * xdr_buf holds responses; the structure fits NFS read responses
  * (header, data pages, optional tail) and enables sharing of
  * client-side routines.
diff --git a/net/sunrpc/svc.c b/net/sunrpc/svc.c
index a8c26634ecf1..ef18365c9d9a 100644
--- a/net/sunrpc/svc.c
+++ b/net/sunrpc/svc.c
@@ -655,6 +655,7 @@ svc_init_buffer(struct svc_rqst *rqstp, const struct svc_serv *serv, int node)
 	}
 
 	rqstp->rq_pages_nfree = rqstp->rq_maxpages;
+	rqstp->rq_next_page = rqstp->rq_respages + rqstp->rq_maxpages;
 	return true;
 }
 
diff --git a/net/sunrpc/svc_xprt.c b/net/sunrpc/svc_xprt.c
index 9d8f6adcfe1f..9b3425f57068 100644
--- a/net/sunrpc/svc_xprt.c
+++ b/net/sunrpc/svc_xprt.c
@@ -686,8 +686,14 @@ static bool svc_alloc_arg(struct svc_rqst *rqstp)
 		rqstp->rq_pages_nfree = 0;
 	}
 
-	if (!svc_fill_pages(rqstp, rqstp->rq_respages, pages))
+	if (WARN_ON_ONCE(rqstp->rq_next_page < rqstp->rq_respages))
 		return false;
+	nfree = rqstp->rq_next_page - rqstp->rq_respages;
+	if (nfree) {
+		if (!svc_fill_pages(rqstp, rqstp->rq_respages, nfree))
+			return false;
+	}
+
 	rqstp->rq_next_page = rqstp->rq_respages;
 	rqstp->rq_page_end = &rqstp->rq_respages[pages];
 	/* svc_rqst_replace_page() dereferences *rq_next_page even
-- 
2.53.0