Re: [RFC 0/2] fuse: introduce fuse server recovery mechanism

[Gao Xiang <hsiangkao@linux.alibaba.com>]

On 2024/5/28 17:32, Christian Brauner wrote:
> On Tue, May 28, 2024 at 05:13:04PM +0800, Gao Xiang wrote:
>> Hi Christian,
>>
>> On 2024/5/28 16:43, Christian Brauner wrote:
>>> On Tue, May 28, 2024 at 12:02:46PM +0800, Gao Xiang wrote:
>>>>
>>>>
>>>> On 2024/5/28 11:08, Jingbo Xu wrote:
>>>>>
>>>>>
>>>>> On 5/28/24 10:45 AM, Jingbo Xu wrote:
>>>>>>
>>>>>>
>>>>>> On 5/27/24 11:16 PM, Miklos Szeredi wrote:
>>>>>>> On Fri, 24 May 2024 at 08:40, Jingbo Xu <jefflexu@linux.alibaba.com> wrote:
>>>>>>>
>>>>>>>> 3. I don't know if a kernel based recovery mechanism is welcome on the
>>>>>>>> community side.  Any comment is welcome.  Thanks!
>>>>>>>
>>>>>>> I'd prefer something external to fuse.
>>>>>>
>>>>>> Okay, understood.
>>>>>>
>>>>>>>
>>>>>>> Maybe a kernel based fdstore (lifetime connected to that of the
>>>>>>> container) would a useful service more generally?
>>>>>>
>>>>>> Yeah I indeed had considered this, but I'm afraid VFS guys would be
>>>>>> concerned about why we do this on kernel side rather than in user space.
>>>>
>>>> Just from my own perspective, even if it's in FUSE, the concern is
>>>> almost the same.
>>>>
>>>> I wonder if on-demand cachefiles can keep fds too in the future
>>>> (thus e.g. daemonless feature could even be implemented entirely
>>>> with kernel fdstore) but it still has the same concern or it's
>>>> a source of duplication.
>>>>
>>>> Thanks,
>>>> Gao Xiang
>>>>
>>>>>>
>>>>>> I'm not sure what the VFS guys think about this and if the kernel side
>>>>>> shall care about this.
>>>
>>> Fwiw, I'm not convinced and I think that's a big can of worms security
>>> wise and semantics wise. I have discussed whether a kernel-side fdstore
>>> would be something that systemd would use if available multiple times
>>> and they wouldn't use it because it provides them with no benefits over
>>> having it in userspace.
>>
>> As far as I know, currently there are approximately two ways to do
>> failover mechanisms in kernel.
>>
>> The first model much like a fuse-like model: in this mode, we should
>> keep and pass fd to maintain the active state.  And currently,
>> userspace should be responsible for the permission/security issues
>> when doing something like passing fds.
>>
>> The second model is like one device-one instance model, for example
>> ublk (If I understand correctly): each active instance (/dev/ublkbX)
>> has their own unique control device (/dev/ublkcX).  Users could
>> assign/change DAC/MAC for each control device.  And failover
>> recovery just needs to reopen the control device with proper
>> permission and do recovery.
>>
>> So just my own thought, kernel-side fdstore pseudo filesystem may
>> provide a DAC/MAC mechanism for the first model.  That is a much
>> cleaner way than doing some similar thing independently in each
>> subsystem which may need DAC/MAC-like mechanism.  But that is
>> just my own thought.
> 
> The failover mechanism for /dev/ublkcX could easily be implemented using
> the fdstore. The fact that they rolled their own thing is orthogonal to
> this imho. Implementing retrieval policies like this in the kernel is
> slowly advancing into /proc/$pid/fd/ levels of complexity. That's all
> better handled with appropriate policies in userspace. And cachefilesd
> can similarly just stash their fds in the fdstore.

Ok, got it.  I just would like to know what kernel fdstore
currently sounds like (since Miklos mentioned it so I wonder
if it's feasible since it can benefit to non-fuse cases).
I think userspace fdstore works for me (unless some other
interesting use cases for evaluation later).

Jingbo has an internal requirement for fuse, that is a pure
fuse stuff, and that is out of my scope though.

Thanks,
Gao Xiang