linux-kernel - Re: [Bug fix] nfs-client: fix nfs_inode_attrs_need_update for async read_done comes during truncating to smaller size

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Date:	Tue, 16 Oct 2012 02:51:35 +0000
From:	"Myklebust, Trond" <Trond.Myklebust@...app.com>
To:	Chen Gang <gang.chen@...anux.com>
CC:	Jeff Layton <jlayton@...hat.com>,
	"linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [Bug fix] nfs-client: fix nfs_inode_attrs_need_update for async
 read_done comes during truncating to smaller size

On Tue, 2012-10-16 at 09:37 +0800, Chen Gang wrote:
> 于 2012年10月15日 20:32, Myklebust, Trond 写道:
> > RPC is not ordered. The fact that we get one RPC reply before another
> > does not mean that the server sent them in that order.
> > 
> > This is doubly true when you use UDP as the transport protocol.
> 
> 1) is it means: nfs_inode_attrs_need_update need not consider async
> read_done situation ?

I don't understand what you mean. This is mainly about the asynchronous
write situation...

> 2) for correctness, I do not think "nfs_size_to_loff_t(fattr->size) >
> i_size_read(inode)" in nfs_size_need_update is enough. (at least need
> use "!=" instead of '>'), do you think so ?

No... If I did, I would have changed this 15 years ago when I was
writing that code. Nothing here is new... 2.6.27-rc9 has the exact same
heuristics.
It boils down to the rule that if you want to ensure that data is not
_lost_, then you have to ensure that the cached file size is not less
than the true file size.

> 3) another reference:
> 
>   A) for an old kernel version (such as 2.6.27-rc9), no such issue
> (because it did not have nfs_size_need_update).
> 
>   B) the test tools which I use is from the LTP (Linux Test Project),
> they use both udp and tcp to test both the nfsv2 and nfsv3.

So what combinations are failing?

>   C) truly LTP has its limitations: "for stress test, LTP let nfs client
> and server under the same machine, which will cause kernel stable
> issue", but for net test, LTP use different machine (I got our issue
> from LTP net test).

Running the client and server on the same machine is likely to deadlock
due to memory pressure issues. The client needs to be able to _increase_
memory pressure on the server in order to reduce its own pressure. That
doesn't work well when client == server.

-- 
Trond Myklebust
Linux NFS client maintainer

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