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From:   Matt Benjamin <mbenjami@redhat.com>
Date:   Tue, 3 Aug 2021 21:43:17 -0400
Message-ID: <CAKOnarkuUpd6waieqvWxJDRpLojwXqbNtAFz_bz6Q2k9Xrskgw@mail.gmail.com>
Subject: Re: cto changes for v4 atomic open
To:     "bfields@fieldses.org" <bfields@fieldses.org>
Cc:     Trond Myklebust <trondmy@hammerspace.com>,
        "plambri@redhat.com" <plambri@redhat.com>,
        "linux-nfs@vger.kernel.org" <linux-nfs@vger.kernel.org>,
        "bcodding@redhat.com" <bcodding@redhat.com>
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I think it is how close-to-open has been traditionally understood.  I
do not believe that close-to-open in any way implies a single writer,
rather it sets the consistency expectation for all readers.

Matt

On Tue, Aug 3, 2021 at 5:36 PM bfields@fieldses.org
<bfields@fieldses.org> wrote:
>
> On Tue, Aug 03, 2021 at 09:07:11PM +0000, Trond Myklebust wrote:
> > On Tue, 2021-08-03 at 16:30 -0400, J. Bruce Fields wrote:
> > > 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.
> > >
> >
> > I'm not following your logic.
>
> It's just a question of what every source I can find says close-to-open
> means.  E.g., NFS Illustrated, p. 248, "Close-to-open consistency
> provides a guarantee of cache consistency at the level of file opens and
> closes.  When a file is closed by an application, the client flushes any
> cached changs to the server.  When a file is opened, the client ignores
> any cache time remaining (if the file data are cached) and makes an
> explicit GETATTR call to the server to check the file modification
> time."
>
> > The close-to-open model assumes that the file is only being modified by
> > one client at a time and it assumes that file contents may be cached
> > while an application is holding it open.
> > The point checks exist in order to detect if the file is being changed
> > when the file is not open.
> >
> > Linux does not have a per-application cache. It has a page cache that
> > is shared among all applications. It is impossible for two applications
> > to open the same file using buffered I/O, and yet see different
> > contents.
>
> Right, so based on the descriptions like the one above, I would have
> expected both applications to see new data at that point.
>
> Maybe that's not practical to implement.  It'd be nice at least if that
> was explicit in the documentation.
>
> --b.
>


-- 

Matt Benjamin
Red Hat, Inc.
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