Re: [RFC] NFSd laundromat containerization

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On Mon, 14 May 2012 13:00:17 +0400
Stanislav Kinsbursky <skinsbursky@xxxxxxxxxxxxx> wrote:

> On 12.05.2012 18:16, bfields@xxxxxxxxxxxx wrote:
> > On Sat, May 12, 2012 at 12:59:05PM +0400, Stanislav Kinsbursky wrote:
> >> On 11.05.2012 17:53, bfields@xxxxxxxxxxxx wrote:
> >>> On Fri, May 11, 2012 at 05:50:44PM +0400, Stanislav Kinsbursky wrote:
> >>>> Hello.
> >>>> I'm currently looking on NFSd laundromat work, and it looks like
> >>>> have to be performed per networks namespace context.
> >>>> It's easy to make corresponding delayed work per network namespace
> >>>> and thus gain per-net data pointer in laundromat function.
> >>>> But here a problem appears: network namespace is required to skip
> >>>> clients from other network namespaces while iterating over global
> >>>> lists (client_lru and friends).
> >>>> I see two possible solutions:
> >>>> 1) Make these list per network namespace context. In this case
> >>>> network namespace will not be required - per-net data will be
> >>>> enough.
> >>>> 2) Put network namespace link on per-net data (this one is easier, but uglier).
> >>>
> >>> I'd rather there be as few shared data structures between network
> >>> namespaces as possible--I think that will simplify things.
> >>>
> >>> So, of those two choices, #1.
> >>>
> >>
> >> Guys, I would like to discuss few ideas about caches and lists containerization.
> >> Currently, it look to me, that these hash tables:
> >>
> >> reclaim_str, conf_id, conf_str, unconf_str, unconf_id, sessionid
> >>
> >> and these lists:
> >>
> >> client_lru, close_lru
> >>
> >> have to be per net, while hash tables
> >>
> >> file, ownerstr, lockowner_ino
> >>
> >> and
> >>
> >> del_recall_lru lists
> >>
> >> have not, because they are about file system access.
> >
> > Actually, ownerstr and lockowner_ino should also both be per-container.
> >
> > So it's only file and del_recall_lru that should be global.  (And
> > del_recall_lru might work either way, actually.)
> >
> >> If I'd containerize it this way, then looks like nfs4_lock_state()
> >> and nfs4_unlock_state() functions will protect only
> >> non-containerized data, while containerized data have to protected
> >> by some per-net lock.
> >
> > Sounds about right.
> >
> 
> Bruce, Jeff, I've implemented these per-net hashes and lists (file hash and 
> del_recall_lru remains global).
> But now I'm confused with locking.
> 
> For example, let's consider file hash and del_recall_lru lists.
> It looks like they are protected by recall_lock. But in 
> nfsd_forget_delegations() this lock is not taken. Is it a bug?

It looks like a bug to me. If another thread is modifying the
file_hashtbl while you're calling nfsd_forget_delegations, then you
could oops here.

Perhaps we only ever modify that table while holding the state mutex in
which case the code won't oops, but the recall lock seems rather
superfluous at that point.

I'd have to unwind the locking and see...

> If yes and recall_lock is used for file hash protection, then why do we need to 
> protect nfsd_process_n_delegations() by nfs4_un/lock_state() calls?
> 
> Actually, the problem I'm trying to solve is to replace global client_lock by 
> per-net one where possible. But I don't clearly understand, what it protects.
> 
> Could you, guys, clarify the state locking to me.
> 

I wish I could -- I'm still wrapping my brain around it too...

-- 
Jeff Layton <jlayton@xxxxxxxxxx>
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