[PATCH] netfs: Fix read-retry for fs with no ->prepare_read()

[David Howells <dhowells@xxxxxxxxxx>]

Fix netfslib's read-retry to only call ->prepare_read() in the backing
filesystem such a function is provided.  We can get to this point if a
there's an active cache as failed reads from the cache need negotiating
with the server instead.

Fixes: ee4cdf7ba857 ("netfs: Speed up buffered reading")
Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
cc: Jeff Layton <jlayton@xxxxxxxxxx>
cc: netfs@xxxxxxxxxxxxxxx
cc: linux-fsdevel@xxxxxxxxxxxxxxx
---
 fs/netfs/read_retry.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/netfs/read_retry.c b/fs/netfs/read_retry.c
index 21b4a54e545e..16b676c68dcd 100644
--- a/fs/netfs/read_retry.c
+++ b/fs/netfs/read_retry.c
@@ -152,7 +152,8 @@ static void netfs_retry_read_subrequests(struct netfs_io_request *rreq)
 			BUG_ON(!len);
 
 			/* Renegotiate max_len (rsize) */
-			if (rreq->netfs_ops->prepare_read(subreq) < 0) {
+			if (rreq->netfs_ops->prepare_read &&
+			    rreq->netfs_ops->prepare_read(subreq) < 0) {
 				trace_netfs_sreq(subreq, netfs_sreq_trace_reprep_failed);
 				__set_bit(NETFS_SREQ_FAILED, &subreq->flags);
 			}