nfs: only reset desc->pg_mirror_idx when mirroring is supported

so that we don't reset desc->pg_mirror_idx for read unnecessarily.
Remove WARN_ON_ONCE from __nfs_pageio_add_request to allow LD to
set pg_mirror_idx for read where pg_mirror_count is always 1.
Signed-off-by: Peng Tao <tao.peng@primarydata.com>
Signed-off-by: Tom Haynes <loghyr@primarydata.com>

fs/nfs/internal.h

@@ -6,6 +6,7 @@
 #include <linux/mount.h>
 #include <linux/security.h>
 #include <linux/crc32.h>
+#include <linux/nfs_page.h>
 
 #define NFS_MS_MASK (MS_RDONLY|MS_NOSUID|MS_NODEV|MS_NOEXEC|MS_SYNCHRONOUS)
 
@@ -261,6 +262,12 @@ static inline void nfs_iocounter_init(struct nfs_io_counter *c)
 	atomic_set(&c->io_count, 0);
 }
 
+static inline bool nfs_pgio_has_mirroring(struct nfs_pageio_descriptor *desc)
+{
+	WARN_ON_ONCE(desc->pg_mirror_count < 1);
+	return desc->pg_mirror_count > 1;
+}
+
 /* nfs2xdr.c */
 extern struct rpc_procinfo nfs_procedures[];
 extern int nfs2_decode_dirent(struct xdr_stream *,

fs/nfs/pagelist.c

@@ -1021,8 +1021,6 @@ static int __nfs_pageio_add_request(struct nfs_pageio_descriptor *desc,
 	unsigned int bytes_left = 0;
 	unsigned int offset, pgbase;
 
-	WARN_ON_ONCE(desc->pg_mirror_idx >= desc->pg_mirror_count);
-
 	nfs_page_group_lock(req, false);
 
 	subreq = req;
@@ -1162,6 +1160,7 @@ int nfs_pageio_add_request(struct nfs_pageio_descriptor *desc,
 		} else
 			dupreq = req;
 
+		if (nfs_pgio_has_mirroring(desc))
 			desc->pg_mirror_idx = midx;
 		if (!nfs_pageio_add_request_mirror(desc, dupreq))
 			return 0;
@@ -1181,6 +1180,7 @@ static void nfs_pageio_complete_mirror(struct nfs_pageio_descriptor *desc,
 	struct nfs_pgio_mirror *mirror = &desc->pg_mirrors[mirror_idx];
 	u32 restore_idx = desc->pg_mirror_idx;
 
+	if (nfs_pgio_has_mirroring(desc))
 		desc->pg_mirror_idx = mirror_idx;
 	for (;;) {
 		nfs_pageio_doio(desc);