[RFC PATCH 5/6] sunrpc: Track consumed rq_pages entries

[Chuck Lever <cel@kernel.org>]

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

Transports that consume pages from the rq_pages array --
svc_tcp_save_pages() for TCP fragment reassembly and
svc_rdma_clear_rqst_pages() for RDMA Read I/O -- NULL
the consumed entries starting at rq_pages[0]. A new
rq_pages_nfree field in struct svc_rqst records the
count of entries consumed.

svc_alloc_arg() uses rq_pages_nfree to refill only
the consumed entries rather than scanning the full
rq_pages array. In steady state, the transport consume
path NULLs a handful of entries per RPC, so the
allocator visits only those entries instead of the
full ~259 slots (for 1MB messages).

svc_init_buffer() initializes rq_pages_nfree to
rq_maxpages so that the first svc_alloc_arg() call
populates every entry.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
---
 include/linux/sunrpc/svc.h        |  8 ++++++++
 net/sunrpc/svc.c                  |  1 +
 net/sunrpc/svc_xprt.c             | 11 ++++++++---
 net/sunrpc/svcsock.c              |  1 +
 net/sunrpc/xprtrdma/svc_rdma_rw.c |  1 +
 5 files changed, 19 insertions(+), 3 deletions(-)

diff --git a/include/linux/sunrpc/svc.h b/include/linux/sunrpc/svc.h
index b1fb728724f5..bb2029e396db 100644
--- a/include/linux/sunrpc/svc.h
+++ b/include/linux/sunrpc/svc.h
@@ -143,6 +143,13 @@ extern u32 svc_max_payload(const struct svc_rqst *rqstp);
  * server thread needs to allocate more to replace those used in
  * sending.
  *
+ * Transport page-consumption contract:
+ *
+ * Transports that consume rq_pages entries (for TCP fragment
+ * reassembly or RDMA Read I/O) NULL entries starting at
+ * rq_pages[0] and set rq_pages_nfree to the count of entries
+ * consumed. svc_alloc_arg() refills only that many entries.
+ *
  * xdr_buf holds responses; the structure fits NFS read responses
  * (header, data pages, optional tail) and enables sharing of
  * client-side routines.
@@ -201,6 +208,7 @@ struct svc_rqst {
 	struct folio		*rq_scratch_folio;
 	struct xdr_buf		rq_res;
 	unsigned long		rq_maxpages;	/* entries per page array */
+	unsigned long		rq_pages_nfree;		/* NULL rq_pages to refill */
 	struct page *		*rq_pages;
 	struct page *		*rq_respages;	/* Reply buffer pages */
 	struct page *		*rq_next_page; /* next reply page to use */
diff --git a/net/sunrpc/svc.c b/net/sunrpc/svc.c
index 620de9abedbb..a8c26634ecf1 100644
--- a/net/sunrpc/svc.c
+++ b/net/sunrpc/svc.c
@@ -654,6 +654,7 @@ svc_init_buffer(struct svc_rqst *rqstp, const struct svc_serv *serv, int node)
 		return false;
 	}
 
+	rqstp->rq_pages_nfree = rqstp->rq_maxpages;
 	return true;
 }
 
diff --git a/net/sunrpc/svc_xprt.c b/net/sunrpc/svc_xprt.c
index cd38f09c1803..9d8f6adcfe1f 100644
--- a/net/sunrpc/svc_xprt.c
+++ b/net/sunrpc/svc_xprt.c
@@ -675,12 +675,17 @@ static bool svc_fill_pages(struct svc_rqst *rqstp, struct page **pages,
 static bool svc_alloc_arg(struct svc_rqst *rqstp)
 {
 	struct xdr_buf *arg = &rqstp->rq_arg;
-	unsigned long pages;
+	unsigned long pages, nfree;
 
 	pages = rqstp->rq_maxpages;
 
-	if (!svc_fill_pages(rqstp, rqstp->rq_pages, pages))
-		return false;
+	nfree = rqstp->rq_pages_nfree;
+	if (nfree) {
+		if (!svc_fill_pages(rqstp, rqstp->rq_pages, nfree))
+			return false;
+		rqstp->rq_pages_nfree = 0;
+	}
+
 	if (!svc_fill_pages(rqstp, rqstp->rq_respages, pages))
 		return false;
 	rqstp->rq_next_page = rqstp->rq_respages;
diff --git a/net/sunrpc/svcsock.c b/net/sunrpc/svcsock.c
index 10a298f440cc..f9140ac8ed99 100644
--- a/net/sunrpc/svcsock.c
+++ b/net/sunrpc/svcsock.c
@@ -1009,6 +1009,7 @@ static void svc_tcp_save_pages(struct svc_sock *svsk, struct svc_rqst *rqstp)
 		svsk->sk_pages[i] = rqstp->rq_pages[i];
 		rqstp->rq_pages[i] = NULL;
 	}
+	rqstp->rq_pages_nfree = npages;
 }
 
 static void svc_tcp_clear_pages(struct svc_sock *svsk)
diff --git a/net/sunrpc/xprtrdma/svc_rdma_rw.c b/net/sunrpc/xprtrdma/svc_rdma_rw.c
index 4ec2f9ae06aa..cf4a1762b629 100644
--- a/net/sunrpc/xprtrdma/svc_rdma_rw.c
+++ b/net/sunrpc/xprtrdma/svc_rdma_rw.c
@@ -1107,6 +1107,7 @@ static void svc_rdma_clear_rqst_pages(struct svc_rqst *rqstp,
 		head->rc_pages[i] = rqstp->rq_pages[i];
 		rqstp->rq_pages[i] = NULL;
 	}
+	rqstp->rq_pages_nfree = head->rc_page_count;
 }
 
 /**
-- 
2.53.0