Re: Regression in 5.1.20: Reading long directory fails

Benjamin Coddington
2019-09-08T11:39:12Z

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

>> On Sep 6, 2019, at 4:47 PM, Jason L Tibbitts III <tibbs@math.uh.edu>
>> wrote:
>>
>>>>>>> "JBF" == J Bruce Fields <bfields@fieldses.org> 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@oracle.com>
>> 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@oracle.com>
>> Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
>>
>> :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