Re: [PATCH v2] ceph: do not update snapshot context when there is no new snapshot

[Xiubo Li <xiubli@redhat.com>]

On 2/18/22 10:47 AM, xiubli@redhat.com wrote:
> From: Xiubo Li <xiubli@redhat.com>
>
> 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.
>
> The 'invalidate' word is not precise here, acutally it will rebuild
> the snapshot existing contexts or just build none-existing ones,
> rename it to 'rebuild_snapcs'.
>
> URL: https://tracker.ceph.com/issues/44100
> Signed-off-by: Xiubo Li <xiubli@redhat.com>
> ---
>
> Changed in V2:
> - Thanks Zheng's feedback and switched to Zheng's patch.
> - Rename invalidate to rebuild_snapcs.
>
>
>
>   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 dbf34f212596..6d55b8ba79d8 100644
> --- a/fs/ceph/snap.c
> +++ b/fs/ceph/snap.c
> @@ -735,7 +735,8 @@ 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);
>   
> @@ -743,6 +744,7 @@ 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);
> @@ -766,7 +768,7 @@ 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",
> @@ -791,22 +793,30 @@ 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 (rebuild_snapcs && p >= e)

s/rebuild_snapcs/realm_to_rebuild/

This will fix the bug Luís Henriques reported.

I have sent the V3 to fix it. Thanks.

- Xiubo


> +		rebuild_snap_realms(realm_to_rebuild, &dirty_realms);
>   
>   	if (!first_realm)
>   		first_realm = realm;