Re: Regression in 5.1.20: Reading long directory fails

[Benjamin Coddington]

On 6 Sep 2019, at 16:50, Chuck Lever wrote:

> > 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
> > https://www.math.uh.edu/~tibbs/nfs/.  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.

After sprinkling the printk's, we're coming up one word short in the 
receive
buffer.  I think we're not accounting for the xdr pad of buf->pages for 
NFS4
readdir -- but I need to check the RFCs.  Anyone know if v4 READDIR 
results
have to be aligned?

Also need to check just why krb5i is the only auth that cares..

Ben