Subject: Re: why does each NFS mount have (at least) 2 rpc_clnt's ?
From: Trond Myklebust <trond.myklebust@fys.uio.no>
To: Jeff Layton <jlayton@redhat.com>
In-Reply-To: <20100401084620.3475d4c0@barsoom.rdu.redhat.com>
References: <20100401084620.3475d4c0@barsoom.rdu.redhat.com>
Date: Thu, 01 Apr 2010 09:17:49 -0400
Message-ID: <1270127869.3533.12.camel@localhost.localdomain>
Cc: linux-nfs@vger.kernel.org, nfsv4@linux-nfs.org
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On Thu, 2010-04-01 at 08:46 -0400, Jeff Layton wrote: 
> I'm working on an issue in an older kernel where we see occasional
> panics when trying to refresh credentials. Here's the bug in case
> anyone is interested:
> 
>     https://bugzilla.redhat.com/show_bug.cgi?id=572870
> 
> ...I think I understand the problem well enough now. The problem is
> pretty complex, but the issue is that some operations are done using
> credentials from a stateowner associated with a nfs_client, but using
> the rpc_clnt in nfs_server->client. The two can have different
> authtypes if there are a mix of mounts with different authtypes to the
> same server. This problem seems to have been fixed in mainline with the
> introduction of the auth_generic code.
> 
> It leaves me wondering though...what exactly is the reason for having
> two rpc_clients per NFS mount? To clarify, I'm talking about these two,
> which seem to be somewhat redundant:
> 
> nfs_server->client
> nfs_server->nfs_client->cl_rpcclient
> 
> On mount, the nfs4_set_client calls nfs_get_client to search the list
> of nfs_client structs until it finds one that matches the address, port,
> etc of the NFS server. If one isn't found, the kernel creates one using
> whatever authtype was requested for the mount.
> 
> Later, nfs_init_server_rpcclient looks at the rpc_clnt in the
> nfs_client and copies it. If the auth pseudoflavor doesn't match
> however, it creates a new rpc_auth for it.
> 
> What exactly is the point of having two rpc_clnt's? Why not just get
> always use nfs_client->cl_rpcclient instead of nfs_server->client and
> simply have nfs_get_client filter by authtype?
> 

Look again at nfs_init_server_rpcclient(). The pseudoflavour is not the
only thing that is changed. We also change the soft flag and the timeout
properties of the server->client.

The point is that users sometimes want to specify per-mountpoint
transport properties, and so we try to give them that possibility, while
at the same time sharing sockets/rdma connections.

Cheers
  Trond

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