[PATCH v2] ceph: flush the mdlog for filesystem sync

[Xiubo Li <xiubli@redhat.com>]

Before waiting for a request's safe reply, we will send the mdlog
flush request to the relevant MDS. And this will also flush the
mdlog for all the other unsafe requests in the same session, so
we can record the last session and no need to flush mdlog again
in the next loop. But there still have cases that it may send the
mdlog flush requst twice or more, but that should be not often.

URL: https://tracker.ceph.com/issues/55284
Signed-off-by: Xiubo Li <xiubli@redhat.com>
---

V2:
- Fixed possible NULL pointer dereference for the req->r_session


 fs/ceph/mds_client.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/fs/ceph/mds_client.c b/fs/ceph/mds_client.c
index 0da85c9ce73a..4aaa7b14136e 100644
--- a/fs/ceph/mds_client.c
+++ b/fs/ceph/mds_client.c
@@ -5098,6 +5098,7 @@ void ceph_mdsc_pre_umount(struct ceph_mds_client *mdsc)
 static void wait_unsafe_requests(struct ceph_mds_client *mdsc, u64 want_tid)
 {
 	struct ceph_mds_request *req = NULL, *nextreq;
+	struct ceph_mds_session *last_session = NULL, *s;
 	struct rb_node *n;
 
 	mutex_lock(&mdsc->mutex);
@@ -5117,6 +5118,15 @@ static void wait_unsafe_requests(struct ceph_mds_client *mdsc, u64 want_tid)
 			ceph_mdsc_get_request(req);
 			if (nextreq)
 				ceph_mdsc_get_request(nextreq);
+
+			/* send flush mdlog request to MDS */
+			s = req->r_session;
+			if (s && last_session != s) {
+				send_flush_mdlog(s);
+				ceph_put_mds_session(last_session);
+				last_session = ceph_get_mds_session(s);
+			}
+
 			mutex_unlock(&mdsc->mutex);
 			dout("wait_unsafe_requests  wait on %llu (want %llu)\n",
 			     req->r_tid, want_tid);
@@ -5135,6 +5145,7 @@ static void wait_unsafe_requests(struct ceph_mds_client *mdsc, u64 want_tid)
 		req = nextreq;
 	}
 	mutex_unlock(&mdsc->mutex);
+	ceph_put_mds_session(last_session);
 	dout("wait_unsafe_requests done\n");
 }
 
-- 
2.36.0.rc1