SUNRPC: Optimize rq_respages allocation in svc_alloc_arg

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.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

1 files changed, 4 insertions, 0 deletions

diff --git a/include/linux/sunrpc/svc.h b/include/linux/sunrpc/svc.h
index c3399cf64524..669c944eaf7f 100644
--- a/include/linux/sunrpc/svc.h
+++ b/include/linux/sunrpc/svc.h
@@ -152,6 +152,10 @@ extern u32 svc_max_payload(const struct svc_rqst *rqstp);
  * still in transport use, and set rq_pages_nfree to the count.
  * svc_alloc_arg() refills only that many rq_pages 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.