On Sun, Mar 01, 2015 at 11:31:31PM -0500, Trond Myklebust wrote: > On Sun, Mar 1, 2015 at 8:20 PM, Trond Myklebust > <trond.myklebust@xxxxxxxxxxxxxxx> wrote: > > On Sun, Mar 1, 2015 at 8:06 PM, Bruce James Fields <bfields@xxxxxxxxxxxx> wrote: > >> On Sun, Mar 01, 2015 at 07:52:28PM -0500, Trond Myklebust wrote: > >>> Hi Bruce, > >>> > >>> On Sun, Mar 1, 2015 at 2:14 PM, Trond Myklebust > >>> <trond.myklebust@xxxxxxxxxxxxxxx> wrote: > >>> > Hi, > >>> > > >>> > When doing testing of NFSv3 loopback mounts (client and server are on > >>> > the same IP address), I'm seeing a very reproducible hang in which the > >>> > client stops receiving data from the server. The TCP connection is still > >>> > marked as established, and the server appears to continue to receive and > >>> > send data, however the client does not. > >>> > > >>> > So far, I've reproduced on both v4.0-rc1, and the Fedora v3.18.7 kernel. > >>> > > >>> > The reproducer is simply to loopback mount using NFSv3, and then run the > >>> > 'fsx' filesystem exerciser. I'm usually able to trigger the hang with > >>> > "fsx -N 100000 foobar". > >>> > > >>> > I've attached a couple of wireshark trace of a few frames just before > >>> > and during the hang in case it jogs any memories. > >>> > >>> This bug appears to go away when I disable the splice()-based reads by > >>> clearing the RQ_SPLICE_OK flag. > >>> > >>> I noticed that it always involved a combination of a READ and a > >>> truncating SETATTR call. Are you sure that it is safe to share > >>> pagecache pages directly with sendpage() in this way? As far as I can > >>> tell, there is no locking to prevent them from being modified while in > >>> the TCP send queue. > >> > >> This is the stable-pages problem that we've had forever, isn't it? Or > >> is this a different problem? > > > > It is causing the TCP socket to hang, so it goes beyond the usual > > stable pages issue. > > > > Confirming that clearing RQ_SPLICE_OK fixes the issue on all kernel > that I've tested so far. Well, if the problem is a race with truncate then I guess it may have something to do with sending pages that are no longer part of the page cache? I'd think that the get_page() in nfsd_splice_actor would prevent the page being put to any other use until the network layer was done with it, so that at worst the client would see garbage. But I don't begin to understand how truncation actually works.... The zero-copy v3 code has been there since 2002, if I'm reading the history right, so if it's really a fundamental problem with the approach then I wonder how it's survived so long. I haven't tried to reproduce yet. --b. -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
