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SubjectRe: Grace period
10.04.2012 03:26, bfields@fieldses.org пишет:
> On Mon, Apr 09, 2012 at 03:24:19PM +0400, Stanislav Kinsbursky wrote:
>> 07.04.2012 03:40, bfields@fieldses.org пишет:
>>> On Fri, Apr 06, 2012 at 09:08:26PM +0400, Stanislav Kinsbursky wrote:
>>>> Hello, Bruce.
>>>> Could you, please, clarify this reason why grace list is used?
>>>> I.e. why list is used instead of some atomic variable, for example?
>>>
>>> Like just a reference count? Yeah, that would be OK.
>>>
>>> In theory it could provide some sort of debugging help. (E.g. we could
>>> print out the list of "lock managers" currently keeping us in grace.) I
>>> had some idea we'd make those lock manager objects more complicated, and
>>> might have more for individual containerized services.
>>
>> Could you share this idea, please?
>>
>> Anyway, I have nothing against lists. Just was curious, why it was used.
>> I added Trond and lists to this reply.
>>
>> Let me explain, what is the problem with grace period I'm facing
>> right know, and what I'm thinking about it.
>> So, one of the things to be containerized during "NFSd per net ns"
>> work is the grace period, and these are the basic components of it:
>> 1) Grace period start.
>> 2) Grace period end.
>> 3) Grace period check.
>> 3) Grace period restart.
>
> For restart, you're thinking of the fs/lockd/svc.c:restart_grace()
> that's called on aisngal in lockd()?
>
> I wonder if there's any way to figure out if that's actually used by
> anyone? (E.g. by any distro init scripts). It strikes me as possibly
> impossible to use correctly. Perhaps we could deprecate it....
>

Or (since lockd kthread is visible only from initial pid namespace) we can just
hardcode "init_net" in this case. But it means, that this "kill" logic will be
broken if two containers shares one pid namespace, but have separated networks
namespaces.
Anyway, both (this one or Bruce's) solutions suits me.

>> So, the simplest straight-forward way is to make all internal stuff:
>> "grace_list", "grace_lock", "grace_period_end" work and both
>> "lockd_manager" and "nfsd4_manager" - per network namespace. Also,
>> "laundromat_work" have to be per-net as well.
>> In this case:
>> 1) Start - grace period can be started per net ns in
>> "lockd_up_net()" (thus has to be moves there from "lockd()") and
>> "nfs4_state_start()".
>> 2) End - grace period can be ended per net ns in "lockd_down_net()"
>> (thus has to be moved there from "lockd()"), "nfsd4_end_grace()" and
>> "fs4_state_shutdown()".
>> 3) Check - looks easy. There is either svc_rqst or net context can
>> be passed to function.
>> 4) Restart - this is a tricky place. It would be great to restart
>> grace period only for the networks namespace of the sender of the
>> kill signal. So, the idea is to check siginfo_t for the pid of
>> sender, then try to locate the task, and if found, then get sender's
>> networks namespace, and restart grace period only for this namespace
>> (of course, if lockd was started for this namespace - see below).
>
> If it's really the signalling that's the problem--perhaps we can get
> away from the signal-based interface.
>
> At least in the case of lockd I suspect we could.
>

I'm ok with that. So, if no objections will follow, I'll drop it and send the
patch. Or you want to do it?

BTW, I tried this "pid from siginfo" approach yesterday. And it doesn't work,
because sender usually dead already, when lookup for task by pid is performed.

> Or perhaps the decision to share a single lockd thread (or set of nsfd
> threads) among multiple network namespaces was a poor one. But I
> realize multithreading lockd doesn't look easy.
>

This decision was the best one in current circumstances.
Having Lockd thread (or NFSd threads) per container looks easy to implement on
first sight. But kernel threads currently supported only in initial pid
namespace. I.e. it means that per-container kernel thread won't be visible in
container, if it has it's own pid namespace. And there is no way to put a kernel
thread into container.
In OpenVZ we have per-container kernel threads. But integrating this feature to
mainline looks hopeless (or very difficult) to me. At least for now.
So this problem with signals remains unsolved.

So, as it looks to me, this "one service per all" is the only one suitable for
now. But there are some corner cases which have to be solved.

Anyway, Jeff's question is still open.
Do we need to prevent people from exporting nested directories from different
network namespaces?
And if yes, how to do this?

--
Best regards,
Stanislav Kinsbursky
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