Re: Regression in 5.1.20: Reading long directory fails

From: Chuck Lever
Date: Fri Sep 06 2019 - 16:53:28 EST

> On Sep 6, 2019, at 4:47 PM, Jason L Tibbitts III <tibbs@xxxxxxxxxxx> wrote:
>>>>>> "JBF" == J Bruce Fields <bfields@xxxxxxxxxxxx> writes:
> JBF> Those readdir changes were client-side, right? Based on that I'd
> JBF> been assuming a client bug, but maybe it'd be worth getting a full
> JBF> packet capture of the readdir reply to make sure it's legit.
> I have been working with bcodding on IRC for the past couple of days on
> this. Fortunately I was able to come up with way to fill up a directory
> in such a way that it will fail with certainty and as a bonus doesn't
> include any user data so I can feel OK about sharing packet captures. I
> have a capture alongside a kernel trace of the problematic operation in
> Not that I can particularly tell
> anything useful from that, but bcodding says that it seems to point to
> some issue in sunrpc.
> And because I can easily reproduce this and I was able to do a bisect:
> 2c94b8eca1a26cd46010d6e73a23da5f2e93a19d is the first bad commit
> commit 2c94b8eca1a26cd46010d6e73a23da5f2e93a19d
> Author: Chuck Lever <chuck.lever@xxxxxxxxxx>
> Date: Mon Feb 11 11:25:41 2019 -0500
> SUNRPC: Use au_rslack when computing reply buffer size
> au_rslack is significantly smaller than (au_cslack << 2). Using
> that value results in smaller receive buffers. In some cases this
> eliminates an extra segment in Reply chunks (RPC/RDMA).
> Signed-off-by: Chuck Lever <chuck.lever@xxxxxxxxxx>
> Signed-off-by: Anna Schumaker <Anna.Schumaker@xxxxxxxxxx>
> :040000 040000 d4d1ce2fbe0035c5bd9df976b8c448df85dcb505 7011a792dfe72ff9cd70d66e45d353f3d7817e3e M net
> But of course, I can't say whether this is the actual bad commit or
> whether it just introduced a behavior change which alters the conditions
> under which the problem appears.

The first place I'd start looking is the XDR constants at the head of fs/nfs/nfs4xdr.c
having to do with READDIR.

The report of behavior changes with the use of krb5p also makes this commit plausible.

> And just to make sure that the blame doesn't lie with the old RHEL7
> kernel, I rsynced over the problematic directory to a machine running
> something slightly more modern (5.1.11, which I know I need to update,
> but it's already set up to do kerberised NFS) and the same problem
> exists, though the directory listing does fail at a different place.
> - J<

Chuck Lever