[PATCH 3/3] ceph: do no update snapshot context when there is no new snapshot

[xiubli@redhat.com]

From: Xiubo Li <xiubli@redhat.com>

No need to update snapshot context when any of the following two
cases happens:
1: if my context seq matches realm's seq and realm has no parent.
2: if my context seq equals or is larger than my parent's, this
   works because we rebuild_snap_realms() works _downward_ in
   hierarchy after each update.

This fix will avoid those inodes which accidently calling
ceph_queue_cap_snap() and make no sense, for exmaple:

There have 6 directories like:

/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. And every time when
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 then the mds will send kclient a snapshot backtrace
in _downward_ in hierarchy: seqs 4, 3. Then in ceph_update_snap_trace()
it will always rebuild the from the last realm, that's the root_snap.
So later when rebuilding the snap context it will always rebuild
the snap_X2 realm and then try to queue cap snaps for all the inodes
related in snap_X2 realm, and we are seeing the logs like:

"ceph:  queue_cap_snap 00000000a42b796b nothing dirty|writing"

URL: https://tracker.ceph.com/issues/44100
Signed-off-by: Xiubo Li <xiubli@redhat.com>
---
 fs/ceph/snap.c | 16 +++++++++-------
 1 file changed, 9 insertions(+), 7 deletions(-)

diff --git a/fs/ceph/snap.c b/fs/ceph/snap.c
index d075d3ce5f6d..1f24a5de81e7 100644
--- a/fs/ceph/snap.c
+++ b/fs/ceph/snap.c
@@ -341,14 +341,16 @@ static int build_snap_context(struct ceph_snap_realm *realm,
 		num += parent->cached_context->num_snaps;
 	}
 
-	/* do i actually need to update?  not if my context seq
-	   matches realm seq, and my parents' does to.  (this works
-	   because we rebuild_snap_realms() works _downward_ in
-	   hierarchy after each update.) */
+	/* do i actually need to update? No need when any of the following
+	 * two cases:
+	 * #1: if my context seq matches realm's seq and realm has no parent.
+	 * #2: if my context seq equals or is larger than my parent's, this
+	 *     works because we rebuild_snap_realms() works _downward_ in
+	 *     hierarchy after each update.
+	 */
 	if (realm->cached_context &&
-	    realm->cached_context->seq == realm->seq &&
-	    (!parent ||
-	     realm->cached_context->seq >= parent->cached_context->seq)) {
+	    ((realm->cached_context->seq == realm->seq && !parent) ||
+	     (parent && realm->cached_context->seq >= parent->cached_context->seq))) {
 		dout("build_snap_context %llx %p: %p seq %lld (%u snaps)"
 		     " (unchanged)\n",
 		     realm->ino, realm, realm->cached_context,
-- 
2.27.0