Re: [PATCH 0/2] NFS: limit use of ACCESS cache for negative responses

[Trond Myklebust <trondmy@hammerspace.com>]

On Tue, 2022-09-20 at 08:38 +1000, NeilBrown wrote:
> On Tue, 20 Sep 2022, Benjamin Coddington wrote:
> > On 26 Aug 2022, at 20:52, Trond Myklebust wrote:
> > 
> > > Can we please try to solve the real problem first? The real
> > > problem is
> > > not that user permissions change every hour on the server.
> > > 
> > > POSIX normally only expects changes to happen to your group
> > > membership
> > > when you log in. The problem here is that the NFS server is
> > > updating
> > > its rules concerning your group membership at some random time
> > > after
> > > your log in on the NFS client.
> > > 
> > > So how about if we just do our best to approximate the POSIX
> > > rules, and
> > > promise to revalidate your cached file access permissions at
> > > least once
> > > after you log in? Then we can let the NFS server do whatever the
> > > hell
> > > it wants to do after that.
> > > IOW: If the sysadmin changes the group membership for the user,
> > > then
> > > the user can remedy the problem by logging out and then logging
> > > back in
> > > again, just like they do for local filesystems.
> > 
> > This goes a long way toward fixing things up for us, I appreciate
> > it, and
> > hope to see it merged.  The version on your testing branch
> > (d84b059f) can
> > have my:
> > 
> > Reviewed-by: Benjamin Coddington <bcodding@redhat.com>
> > Tested-by: Benjamin Coddington <bcodding@redhat.com>
> > 
> 
> The test in that commit can be "gamed".
> I could write a tool that double-forks with the intermediate exiting
> so the grandchild will be inherited by init.  Then the grandchild can
> access the problematic path and force the access cache for the
> current
> user to be refreshed.  It would optionally need to do a 'find' to be
> thorough.

Sure... If two tasks share the same cred, one can do deliberately crazy
stuff to break performance of the other. Is that news?

-- 
Trond Myklebust
Linux NFS client maintainer, Hammerspace
trond.myklebust@hammerspace.com