Re: NFSv4.1 session reset needs to update ->rsize and ->wsize - how???

[NeilBrown <neilb@suse.com>]

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On Tue, Sep 04 2018, Trond Myklebust wrote:

> On Wed, 2018-09-05 at 08:47 +1000, NeilBrown wrote:
>> With NFSv4.1, the server specifies max_rqst_sz and max_resp_sz in the
>> reply to CREATE session.
>> 
>> If the client finds it needs to call nfs4_reset_session(), it might
>> get
>> smaller sizes back, so any pending read/writes would need to be
>> resized.
>> 
>> However, I cannot see how the retry handling for reads/writes has any
>> chance to change the size.  It looks like a request is broken up to
>> match the original ->rsize and ->wsize, then those individual IO
>> requests can be retried, but the higher level request is never
>> re-evaluated in light of a new size.
>> 
>> Am I missing something, or is this not supported at present?
>> If it isn't supported, any suggestions on how best to handle a
>> reduction of the rsize/wsize ??
>> 
>
> Why would a sane server want to do this?

Why would a sane protocol support it? :-)

I have a network trace of SLE11-SP4 (3.0 based) talking to "a NetApp
appliance".
It sends a 64K write and gets NFS4ERR_REQ_TOO_BIG.
It then closes the file (getting NFS4ERR_SEQ_MISORDERED even though it
used a seq number 1 more than the WRITE request), and then
DESTROY_SESSION and CREATE_SESSION.
The CREATE_SESSION gets "max req size" of 33812 and "max resp size" of
33672.
It then opens the file again and retries the 64K write....

I have a separate trace showing the initial mount where the sizes are 71680
and 81920.

I don't have a trace where it stops working, but reportedly writes work
smoothly for some hours after a mount, but then suddenly stop working.

The CREATE_SESSION *call* requests I see have the small (32K) sizes, but
presumably they are the result of a previous CREATE_SESSION reply giving
a small value.

I just had a thought.
If one session is shared by two "struct nfs_server" with different
->rsize or ->wsize, then the session might get set up with the smaller
size, and the mount using the larger size will get confused.
In 3.0 (and even 3.10) nfs4_init_session() limits the requested session
parameters to ->rsize and ->wsize.
That changed in 18aad3d552c7.

Maybe I just need to remove that code from nfs4_init_session().
I'll give it a try.

Thanks,
NeilBrown

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