Re: cto changes for v4 atomic open

[bfields@fieldses.org (J. Bruce Fields)]

On Fri, Jul 30, 2021 at 02:48:41PM +0000, Trond Myklebust wrote:
> On Fri, 2021-07-30 at 09:25 -0400, Benjamin Coddington wrote:
> > I have some folks unhappy about behavior changes after: 479219218fbe
> > NFS:
> > Optimise away the close-to-open GETATTR when we have NFSv4 OPEN
> > 
> > Before this change, a client holding a RO open would invalidate the
> > pagecache when doing a second RW open.
> > 
> > Now the client doesn't invalidate the pagecache, though technically
> > it could
> > because we see a changeattr update on the RW OPEN response.
> > 
> > I feel this is a grey area in CTO if we're already holding an open. 
> > Do we
> > know how the client ought to behave in this case?  Should the
> > client's open
> > upgrade to RW invalidate the pagecache?
> > 
> 
> It's not a "grey area in close-to-open" at all. It is very cut and
> dried.
> 
> If you need to invalidate your page cache while the file is open, then
> by definition you are in a situation where there is a write by another
> client going on while you are reading. You're clearly not doing close-
> to-open.

Documentation is really unclear about this case.  Every definition of
close-to-open that I've seen says that it requires a cache consistency
check on every application open.  I've never seen one that says "on
every open that doesn't overlap with an already-existing open on that
client".

They *usually* also preface that by saying that this is motivated by the
use case where opens don't overlap.  But it's never made clear that
that's part of the definition.

--b.