linux-kernel - Re: [Devel] [PATCH 2/6] nfsd: swap fs root in NFSd kthreads

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Message-ID: <20130111170312.GF17909@fieldses.org>
Date:	Fri, 11 Jan 2013 12:03:12 -0500
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	Stanislav Kinsbursky <skinsbursky@...allels.com>
Cc:	linux-nfs@...r.kernel.org, linux-kernel@...r.kernel.org,
	devel@...nvz.org
Subject: Re: [Devel] [PATCH 2/6] nfsd: swap fs root in NFSd kthreads

On Fri, Jan 11, 2013 at 06:56:58PM +0400, Stanislav Kinsbursky wrote:
> 11.12.2012 19:35, J. Bruce Fields пишет:
> >On Tue, Dec 11, 2012 at 10:20:36AM -0500, J. Bruce Fields wrote:
> >>On Tue, Dec 11, 2012 at 07:07:00PM +0400, Stanislav Kinsbursky wrote:
> >>>I don't really understand, how  mountd's root can be wrong. I.e.
> >>>its' always right as I see it. NFSd kthreads have to swap/use
> >>>relative path/whatever to communicate with proper mountd.
> >>>Or I'm missing something?
> >>
> >>Ugh, I see the problem: I thought svc_export_request was called at the
> >>time mountd does the read, but instead its done at the time nfsd does
> >>the upcall.
> >>
> >>I suspect that's wrong, and we really want this done in the context of
> >>the mountd process when it does the read call.  If d_path is called
> >>there then we have no problem.
> >
> >Right, so I'd be happier if we could modify sunrpc_cache_pipe_upcall to
> >skip calling cache_request and instead delay that until cache_read().  I
> >think that should be possible.
> >
> 
> So, Bruce, what we going to do (or what you want me to do) with the rest of NFSd changes?
> I.e. how I should solve this d_path() problem?
> I.e. I don't understand what did you mean by "I'd be happier if we could modify sunrpc_cache_pipe_upcall to
> skip calling cache_request and instead delay that until cache_read()".
> Could you give me a hint?

Definitely.  So normally the way these upcalls happen are:

	1. the kernel does a cache lookup, finds no matching item, and
	   calls sunrpc_cache_pipe_upcall().
	2. sunrpc_cache_pipe_upcall() formats the upcall: it allocates a
	   struct cache_request crq and fills crq->buf with the upcall
	   data by calling the cache's ->cache_request() method.
	3. Then rpc.mountd realizes there's data available in
	   /proc/net/rpc/nfsd.fh/content, so it does a read on that file.
	4. cache_read copies the formatted upcall from crq->buf to
	   to userspace.

So all I'm suggesting is that instead of calling ->cache_request() at
step 2, we do it at step 4.

Then cache_request will be called from rpc.mountd's read.  So we'll know
which container rpc.mountd is in.

Does that make sense?

--b.
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