[linux-cifs-client] linux-cifs-client Digest, Vol 70, Issue 25

[Jeff Layton]

On Mon, 28 Sep 2009 09:41:08 -0500
"Steve French (smfltc)" <smfltc at us.ibm.com> wrote:

> 
> >> This patchset is still preliminary and is just an RFC...
> >>
> >> First, some background. When I was at Connectathon this year, Trond
> >> mentioned an interesting idea to me. He said (paraphrasing):
> >>
> >> "Why doesn't CIFS just use the RPC layer for transport? It's very
> >> efficient at shoveling bits out onto the wire. You'd just need to
> >> abstract out the XDR/RPC specific bits."
> >>
> >>     
> My first reaction is that if you abstract out XDR/RPC specific parts of 
> SunRPC it isn't SunRPC,
> just a scheduler on top of tcp (not a bad thing in theory).   Pulling 
> out the two key pieces from
> SunRPC:
>     - asynchronous event handling and scheduling
>     - upcall for credentials
> could be useful, but does add a lot of complexity.   If there is a way 
> to use just the async
> scheduling (and perhaps upcall) out of SunRPC, that part sounds fine as 
> long as it
> can skip the encoding/decoding and just pass in a raw kvec containing 
> the SMB
> header and data.
> 

Well, the sunrpc layer currently contains a lot of pieces:

1) client side call/response handling (clnt routines)
2) server side call/response handling (svc routines)
3) XDR encoding and decoding routines (including crypto signatures, etc)

...the idea is to hook up new encoding and decoding routines and to add
a new "transport class" which will make the client-side scheduler handle
SMB/SMB2 properly.

We'll also eventually have to add new authentication/credential
"classes" too. I haven't researched that yet in any real depth, so I
can't state much about how difficult it'll be.
 
> >>
> >> CIFS in particular is also designed around synchronous ops, which
> >> seriously limits throughput. Retrofitting it for asynchronous operation
> >> will be adding even more kludges. 
> >>     
> There are only three operations that we can send asynchronous today, all 
> of which require
> special case handling in the VFS already:
>     - readpages
>    - writepages
>    - blocking locks
> (and also directory change notification which we and nfs don't do).   I 
> think the "slow_work"
> mechanism is probably sufficient for these cases already.
> 

The problem is that rolling a mechanism to handle asynchronous ops is
difficult to get right. I think it makes a lot of sense to reuse a
proven engine here. It also makes a lot of sense to implement
synchronous ops on top of an asynchronous infrastructure. RPC does this
under the hood, and so did smbfs.

What you're proposing, in effect, is to do this in reverse -- implement
an asynchronous transport engine using synchronous ops and offloading
the background parts onto threads. That's possible I suppose, but it
means you have a lot of tasks sleeping in the kernel and waiting for
stuff to happen.

> >>  works in our favor...
> >> ------------------------------------------------------------------------
> >> Q: can we hook up cifs or smbfs to use this as a transport?
> >>
> >> A: Not trivially. CIFS in particular is not designed with each call
> >> having discrete encode and decode functions. They're sort of mashed
> >>     
> We certainly don't want to move to an abstract encoding mechanism, 
> especially for SMB2
> where there is only one encoding of wire operations, and no duplicate 
> requests due
> to 20 years of dialects. I can see an argument for abstract encoding 
> for requests
> like SMB open, vs. SMB OpenX vs. SMB NTCreateX but this would be harder or
> to abstract and has to be done case by case anyway due to differences in
> field length, missing fields, different compensations.  It is not
> like the simpler NFS case where encoding involves endian conversion etc.
> 

I'm not sure what you mean by this. Assembling an SMB header and call
is very similar to assembling an RPC header and call. There are
differences of course, but they aren't that substantial.

SMB does introduce some more interesting wrinkles. For instance, since
state is tied up with the actual socket connection, we'll probably need
callbacks into the fs for socket state changes. That doesn't have much
to do with how you abstract out the encoding and decoding though.

> >> ------------------------------------------------------------------------
> >> Q: could we use this as a transport layer for a smb2fs ?
> >>
> >> A: Yes, I think so. This particular prototype is build around SMB1, but
> >> SMB2 could be supported with only minor modifications. One of the
> >> reasons for sending this patchset now before I've built a filesystem on
> >> top of it is because I know that SMB2 work is in progress. I'd like to
> >> see it based around a more asynchronous transport model, or at least
> >> built with cleaner layering so that we can eventually bolt on a different
> >> transport layer if we so choose.
> >>     
> Amost all the ops use "send_receive"  already - so there is no need to 
> change the code much above
> that if you want to experiment with changing the transport.   I like the 
> idea of the
> abtraction of async operations, and creating completion routines (and an 
> async send
> abstraction) for readpages,  writepages and directory change 
> notification would make sense.
> but in both cifs and smb2, the 95% of the operations that must be 
> synchronous in
> the VFS (open, lookup, unlink, create etc.) can already be hooked up to 
> any transport
> as long as it can send a kvec contain fs data and return a response 
> (like the "send_receive"
> and equivalent).
> 

The problem with the send_receive interface is that it assumes that the
encoding, send and decoding will be done by the same task. I think that
assumption will greatly limit this code later and force you to rely on
workarounds (like slow_work) to get asynchronous behavior.

At the very least, I suggest splitting off the decode portions into
separate functions. That at least should allow you the ability later to
offload that part to other tasks (similar to how async tasks get
offloaded to rpciod).

> The idea of doing abstract translation and encoding of SMB protocol frames
> does seem overengineered and probably would make it harder to 
> read/understand
> the setup of certain complex request frames which are quite different from
> Samba to Windows.    As another example, generalized, abstract SMB frame
> conversion isn't being done in Samba 3 for example, and with only
> 19 requests in SMB2 it makes even less sense.   On the client, since
> we have control over which types of requests we send, our case
> is simpler than for the server for sending requests, but in
> response processing since we have to work around server bugs, xdr like
> decoding of SMB responses could get harder still.
> 

Again, I don't see SMB as being that different from NFS in this regard.
You have a transport header (similar to the fraghdr with NFS TCP
transport), then a protocol header (the SMB/SMB2 header), and then
call-specific information. RPC/NFS works exactly the same way.

The code I've proposed more or less has abstractions along those lines.
There's:

transport class -- (net/sunrpc/xprtsmb.c)
header encoding/decoding -- (net/sunrpc/smb.c)

...the other parts will be implemented by the filesystem (similar to
how fs/nfs/nfs?xdr.c work).

> I like the idea of the way SunRPC keeps task information, and it may 
> make it easier
> to carry credentials around (although I think using Dave Howell's key 
> management code
> might be ok instead to access Winbind).   I am not sure how easy it 
> would be to tie
> SunRPC credential mapping to Winbind but that could probably be done.  I 
> like the
> async scheduling capability of SunRPC although I suspect that it is a 
> factor in
> a number of the (nfs client) performance problems we have seen so may 
> need more work.
> I don't like adding (in effect) an extra transport and "encoding layer" 
> though to
> protocols (cifs and smb2).   NFS since it is built on SunRPC on the 
> wire, required
> such a layer, and it makes sense for NFS to layer the code, like their 
> protocol,
> over SunRPC.   CIFS and SMB2 don't require (or even allow) XDR translation,
> variable encodings, and SunRPC encapsulation so the idea of abstracting the
> encoding of something that has a single defined encoding seems wrong.

I'm not sure I understand this last comment. CIFS/SMB2 and NFS are just
not that different in this regard. Either way you have to marshal up
the buffer correctly before you send it, and decode it properly.

Is it possible to go too far with layering and abstraction? Sure. I
don't think I've done that here though.

-- 
Jeff Layton <jlayton at redhat.com>