Re: [PATCH v3] ceph: do not update snapshot context when there is no new snapshot

[Xiubo Li <xiubli@redhat.com>]

On 2/22/22 12:43 AM, Luís Henriques wrote:
> xiubli@redhat.com writes:
>
>> 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.
>>
>> Also, the 'invalidate' word is not precise here. In actuality, it will
>> cause a rebuild of the existing snapshot contexts or just build
>> non-existant ones. Rename it to 'rebuild_snapcs'.
>>
>> URL: https://tracker.ceph.com/issues/44100
>> Signed-off-by: Xiubo Li <xiubli@redhat.com>
>> Signed-off-by: Jeff Layton <jlayton@kernel.org>
>> ---
>>
>>
>>
>> V3:
>> - Fixed the crash issue reproduced by Luís.
> Thanks, I can confirm I'm no longer seeing this issue.

Cool, thanks Luis.

- Xiubo

>
> Cheers,