linux-kernel - Re: Regression in 5.1.20: Reading long directory fails

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Message-ID: <A862CFCD-76A2-4373-8F44-F156DB38E6A5@redhat.com>
Date:   Sun, 08 Sep 2019 07:39:08 -0400
From:   "Benjamin Coddington" <bcodding@...hat.com>
To:     "Chuck Lever" <chuck.lever@...cle.com>
Cc:     "Jason L Tibbitts III" <tibbs@...h.uh.edu>,
        "Bruce Fields" <bfields@...ldses.org>,
        "Wolfgang Walter" <linux@...m.de>,
        "Linux NFS Mailing List" <linux-nfs@...r.kernel.org>,
        km@...all.com, linux-kernel@...r.kernel.org
Subject: Re: Regression in 5.1.20: Reading long directory fails

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

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