[PATCH 4.19 041/105] ceph: do not update snapshot context when there is no new snapshot

Greg Kroah-Hartman
2022-12-05T19:19:34Z

From: Xiubo Li <xiubli@redhat.com>

[ Upstream commit 2e586641c950e7f3e7e008404bd783a466b9b590 ]

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>
Stable-dep-of: 51884d153f7e ("ceph: avoid putting the realm twice when decoding snaps fails")
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
fs/ceph/snap.c | 28 +++++++++++++++++++---------
1 file changed, 19 insertions(+), 9 deletions(-)

diff --git a/fs/ceph/snap.c b/fs/ceph/snap.c
index 5cf7b5f4db94..642040beef91 100644
--- a/fs/ceph/snap.c
+++ b/fs/ceph/snap.c
@@ -673,12 +673,14 @@ 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);

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);
@@ -702,7 +704,7 @@ int ceph_update_snap_trace(struct ceph_mds_client *mdsc,
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",
@@ -727,22 +729,30 @@ int ceph_update_snap_trace(struct ceph_mds_client *mdsc,
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;
--
2.35.1