linux-kernel - Re: [RFC PATCH v2] ceph: prevent a client from exceeding the MDS maximum xattr size

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Date:   Fri, 27 May 2022 17:28:02 +0800
From:   Xiubo Li <xiubli@...hat.com>
To:     Luís Henriques <lhenriques@...e.de>,
        Gregory Farnum <gfarnum@...hat.com>
Cc:     Jeff Layton <jlayton@...nel.org>,
        Ilya Dryomov <idryomov@...il.com>,
        ceph-devel <ceph-devel@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH v2] ceph: prevent a client from exceeding the MDS
 maximum xattr size


On 5/27/22 5:14 PM, Luís Henriques wrote:
> Gregory Farnum <gfarnum@...hat.com> writes:
>
>> On Thu, May 26, 2022 at 6:10 PM Xiubo Li <xiubli@...hat.com> wrote:
>>>
>>> On 5/27/22 8:44 AM, Jeff Layton wrote:
>>>> On Fri, 2022-05-27 at 08:36 +0800, Xiubo Li wrote:
>>>>> On 5/27/22 2:39 AM, Jeff Layton wrote:
>>>>>> A question:
>>>>>>
>>>>>> How do the MDS's discover this setting? Do they get it from the mons? If
>>>>>> so, I wonder if there is a way for the clients to query the mon for this
>>>>>> instead of having to extend the MDS protocol?
>>>>> It sounds like what the "max_file_size" does, which will be recorded in
>>>>> the 'mdsmap'.
>>>>>
>>>>> While currently the "max_xattr_pairs_size" is one MDS's option for each
>>>>> daemon and could set different values for each MDS.
>>>>>
>>>>>
>>>> Right, but the MDS's in general don't use local config files. Where are
>>>> these settings stored? Could the client (potentially) query for them?
>>> AFAIK, each process in ceph it will have its own copy of the
>>> "CephContext". I don't know how to query all of them but I know there
>>> have some API such as "rados_conf_set/get" could do similar things.
>>>
>>> Not sure whether will it work in our case.
>>>
>>>> I'm pretty sure the client does fetch and parse the mdsmap. If it's
>>>> there then it could grab the setting for all of the MDS's at mount time
>>>> and settle on the lowest one.
>>>>
>>>> I think a solution like that might be more resilient than having to
>>>> fiddle with feature bits and such...
>>> Yeah, IMO just making this option to be like the "max_file_size" is more
>>> appropriate.
>> Makes sense to me — this is really a property of the filesystem, not a
>> daemon, so it should be propagated through common filesystem state.
> Right now the max_xattr_pairs_size seems to be something that can be set
> on each MDS, so definitely not a filesystem property.  To be honest, I
> think it's nasty to have this knob in the first place because it will
> allow an admin to set it to a value that will allow clients to blowup the
> MDS cluster.
>
>> I guess Luis' https://github.com/ceph/ceph/pull/46357 should be
>> updated to do it that way?
> Just to confirm, by "to do it that way" you mean to move that setting into
> the mdsmap, right?

Yeah, I think so.

-- XIubo

>
>> I see some discussion there about handling
>> old clients which don't recognize these limits as well.
> Yeah, this is where the feature bit came from.  This would allow old
> clients to be identified so that the MDS would not give them 'Xx'
> capabilities.  Old clients would be able to set xattrs but not to buffer
> them, i.e. they'd be forced to do the SETXATTR synchronously.
>
> Cheers,