[linux-cifs-client] Re: [PATCH] cifs: remove dnotify thread code

[Jamie Lokier]

Steve French wrote:
> We really need to revisit the dnotify/inotify code ... the SMB spec
> lists what the protocol allows, but AFAIK no one has had a chance to
> prototype this.   It is among the highest priority features still
> needed to implement in cifs.
> 
> As we have talked about before, dnotify (or inotify) is even more
> useful for network file systems than local file systems (since the
> alternative, polling, is too expensive to do over the network).   CIFS
> and SMB2 protocols, unlike NFS, has a mechanism to handle dnotify, but
> mapping it to the VFS has not been investigated sufficiently

I think NFSv4 can do it with delegations, although the exact semantics
an app could rely on would be different.

There are several different kinds of notify that apps could use for
different purposes: 1. Queued (an event is posted after a change, will
eventually reach the app).  2. Coherent (the app can ask "any
notifications for me" and if the answer is no, it can be sure that an
attribute/data read issued prior to asking would have yielded what the
app already cached; a sort of app cache validation).  3. Lease (the
app is notified and must respond prior to a change being allowed to
proceed).

These form a hierarchy: if the OS/filesystem provides Lease (3), the
app can use it in place of Coherent (2) and Queued (1).  If the
OS/filesystem provides Lease (3) or Coherent (2), the app can use
either in place of Queued (1).

All of these have different, useful applications.  GUIs like Nautilus
are happy with Queued (1).  For content indexers, it depends how you
want them to behave, and how up to date they should seem.  For
something which computes things from file contents or attributes, such
as (possible beneficiaries) any scripting language, Make, Git, JIT
system, or web templating system, it needs Coherent (2) notifications;
Queued (1) is not reliable for caching.  Lease (3) is potentially
useful for distributed databases.

For CIFS/SMB it looks like all three could be implemented, in
different ways.  I'm not sure if NFSv4 can do Queued, but I think with
delegations it can do Coherent and Lease.

For local filesystems, I think Linux provides Coherent but I haven't
looked closely.  If not, it provides Queued.

Al Viro said (long ago) apps should not rely upon timely
dnotify/inotify events, and therefore should not use them for
consistent app caching.  However, timely delivery isn't required,
what's required is that reading the inotify descriptor (or reading a
flag set by delivery of the dnotify signal / inotify SIGIO?) will
definitely return an event if the corresponding file change has become
observable by other means.  It is about ordering guarantees.

If there is a revamp of the fsnotify code for networking especially,
please at least have a little think about the different event delivery
models and what apps can expect, when which semantics to support.  The
different models are each useful, and may involve different parts of
the networked filesystem protocol.

-- Jamie