ceph: do not update snapshot context when there is no new snapshot

We will only track the uppest parent snapshot realm from which we
need to rebuild the snapshot contexts _downward_ in hierarchy. For
all the others having no new snapshot we will do nothing.

This fix will avoid calling ceph_queue_cap_snap() on some inodes
inappropriately. For example, with the code in mainline, suppose there
are 2 directory hierarchies (with 6 directories total), like this:

/dir_X1/dir_X2/dir_X3/
/dir_Y1/dir_Y2/dir_Y3/

Firstly, make a snapshot under /dir_X1/dir_X2/.snap/snap_X2, then make a
root snapshot under /.snap/root_snap. Every time we make snapshots under
/dir_Y1/..., the kclient will always try to rebuild the snap context for
snap_X2 realm and finally will always try to queue cap snaps for dir_Y2
and dir_Y3, which makes no sense.

That's because the snap_X2's seq is 2 and root_snap's seq is 3. So when
creating a new snapshot under /dir_Y1/... the new seq will be 4, and
the mds will send the kclient a snapshot backtrace in _downward_
order: seqs 4, 3.

When ceph_update_snap_trace() is called, it will always rebuild the from
the last realm, that's the root_snap. So later when rebuilding the snap
context, the current logic will always cause it to rebuild the snap_X2
realm and then try to queue cap snaps for all the inodes related in that
realm, even though it's not necessary.

This is accompanied by a lot of these sorts of dout messages:

    "ceph:  queue_cap_snap 00000000a42b796b nothing dirty|writing"

Fix the logic to avoid this situation.

Also, the 'invalidate' word is not precise here. In actuality, it will
cause a rebuild of the existing snapshot contexts or just build
non-existent ones. Rename it to 'rebuild_snapcs'.

URL: https://tracker.ceph.com/issues/44100
Signed-off-by: Xiubo Li <xiubli@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

diff --git a/fs/ceph/snap.c b/fs/ceph/snap.c
index bc5ec72..5fd79ae 100644 (file)
--- a/fs/ceph/snap.c
+++ b/fs/ceph/snap.c
@@ -708,7 +708,8 @@ int ceph_update_snap_trace(struct ceph_mds_client *mdsc,
        __le64 *prior_parent_snaps;        /* encoded */
        struct ceph_snap_realm *realm = NULL;
        struct ceph_snap_realm *first_realm = NULL;
-       int invalidate = 0;
+       struct ceph_snap_realm *realm_to_rebuild = NULL;
+       int rebuild_snapcs;
        int err = -ENOMEM;
        LIST_HEAD(dirty_realms);
 
@@ -716,6 +717,7 @@ int ceph_update_snap_trace(struct ceph_mds_client *mdsc,
 
        dout("update_snap_trace deletion=%d\n", deletion);
 more:
+       rebuild_snapcs = 0;
        ceph_decode_need(&p, e, sizeof(*ri), bad);
        ri = p;
        p += sizeof(*ri);
@@ -739,7 +741,7 @@ more:
        err = adjust_snap_realm_parent(mdsc, realm, le64_to_cpu(ri->parent));
        if (err < 0)
                goto fail;
-       invalidate += err;
+       rebuild_snapcs += err;
 
        if (le64_to_cpu(ri->seq) > realm->seq) {
                dout("update_snap_trace updating %llx %p %lld -> %lld\n",
@@ -764,22 +766,30 @@ more:
                if (realm->seq > mdsc->last_snap_seq)
                        mdsc->last_snap_seq = realm->seq;
 
-               invalidate = 1;
+               rebuild_snapcs = 1;
        } else if (!realm->cached_context) {
                dout("update_snap_trace %llx %p seq %lld new\n",
                     realm->ino, realm, realm->seq);
-               invalidate = 1;
+               rebuild_snapcs = 1;
        } else {
                dout("update_snap_trace %llx %p seq %lld unchanged\n",
                     realm->ino, realm, realm->seq);
        }
 
-       dout("done with %llx %p, invalidated=%d, %p %p\n", realm->ino,
-            realm, invalidate, p, e);
+       dout("done with %llx %p, rebuild_snapcs=%d, %p %p\n", realm->ino,
+            realm, rebuild_snapcs, p, e);
 
-       /* invalidate when we reach the _end_ (root) of the trace */
-       if (invalidate && p >= e)
-               rebuild_snap_realms(realm, &dirty_realms);
+       /*
+        * this will always track the uppest parent realm from which
+        * we need to rebuild the snapshot contexts _downward_ in
+        * hierarchy.
+        */
+       if (rebuild_snapcs)
+               realm_to_rebuild = realm;
+
+       /* rebuild_snapcs when we reach the _end_ (root) of the trace */
+       if (realm_to_rebuild && p >= e)
+               rebuild_snap_realms(realm_to_rebuild, &dirty_realms);
 
        if (!first_realm)
                first_realm = realm;