linux-kernel - Re: [PATCH linux-next v2] SUNRPC: rpcrdma_register_default_external: Dynamically allocate ib_phys

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Date:	Mon, 11 Mar 2013 19:48:51 +0000
From:	"Myklebust, Trond" <Trond.Myklebust@...app.com>
To:	"J. Bruce Fields" <bfields@...ldses.org>
CC:	Tim Gardner <tim.gardner@...onical.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"David S. Miller" <davem@...emloft.net>, "Tom Tucker" <tom@....us>,
	Haggai Eran <haggaie@...lanox.com>,
	Or Gerlitz <ogerlitz@...lanox.com>,
	Shani Michaeli <shanim@...lanox.com>,
	"linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH linux-next v2] SUNRPC:
 rpcrdma_register_default_external: Dynamically allocate ib_phys_buf

On Mon, 2013-03-11 at 15:15 -0400, J. Bruce Fields wrote:
> On Mon, Mar 11, 2013 at 12:51:44PM -0600, Tim Gardner wrote:
> > On 03/11/2013 12:14 PM, J. Bruce Fields wrote:
> > <snip>
> > >>
> > >> v2 - Move the array of 'struct ib_phys_buf' objects into struct rpcrdma_req
> > >> and pass this request down through rpcrdma_register_external() and
> > >> rpcrdma_register_default_external(). This is less overhead then using
> > >> kmalloc() and requires no extra error checking as the allocation burden is
> > >> shifted to the transport client.
> > > 
> > > Oh good--so that works, and the req is the right place to put this?  How
> > > are you testing this?
> > > 
> > > (Just want to make it clear: I'm *not* an expert on the rdma code, so my
> > > suggestion to put this in the rpcrdma_req was a suggestion for something
> > > to look into, not a claim that it's correct.)
> > > 
> > 
> > Just compile tested so far. Incidentally, I've been through the call stack:
> > 
> > call_transmit
> >  xprt_transmit
> >   xprt->ops->send_request(task)
> >    xprt_rdma_send_request
> >     rpcrdma_marshal_req
> >      rpcrdma_create_chunks
> >       rpcrdma_register_external
> >        rpcrdma_register_default_external
> > 
> > It appears that the context for kmalloc() should be fine unless there is
> > a spinlock held around call_transmit() (which seems unlikely).
> 
> Right, though I think it shouldn't be GFP_KERNEL--looks like writes
> could wait on it.

Nothing inside the RPC client should be using anything heavier than
GFP_NOWAIT (unless done at setup).

> In any case, the embedding-in-rpcrdma_req solution does look cleaner if
> that's correct (e.g. if we can be sure there won't be two simultaneous
> users of that array).

Putting it in the rpcrdma_req means that you have one copy per transport
slot. Why not rather put it in the rpcrdma_xprt?
AFAICS you only need this array at transmit time for registering memory
for RDMA, at which time the transport XPRT_LOCK guarantees that nobody
else is competing for these resources.

-- 
Trond Myklebust
Linux NFS client maintainer

NetApp
Trond.Myklebust@...app.com
www.netapp.com
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