Re: [PATCH] nfs: open-associated setattr shouldn't invalidate own cache

["J. Bruce Fields" <bfields@fieldses.org>]

On Tue, Nov 01, 2011 at 08:43:25PM -0400, Jeff Layton wrote:
> On Tue, 1 Nov 2011 16:07:27 -0700
> "Myklebust, Trond" <Trond.Myklebust@netapp.com> wrote:
> 
> > > -----Original Message-----
> > > From: J. Bruce Fields [mailto:bfields@fieldses.org]
> > > Sent: Tuesday, November 01, 2011 4:27 PM
> > > To: Myklebust, Trond
> > > Cc: J. Bruce Fields; Myklebust, Trond; linux-nfs@vger.kernel.org
> > > Subject: Re: [PATCH] nfs: open-associated setattr shouldn't invalidate
> > own
> > > cache
> > > 
> > > On Mon, Jul 18, 2011 at 08:09:15PM -0400, Myklebust, Trond wrote:
> > > > We should already optimize away the unnecessary setting of the size.
> > > 
> > > Do you remember what commit fixed that?  (Was it an nfs change or a
> > vfs
> > > change?)
> > 
> > It predates the git repository. See the comment about "Optimization:" in
> > nfs_setattr().
> > 
> > > > The problem is that truncate() still requires you to set the ctime,
> > whereas
> > > ftruncate() does not iirc.
> > > 
> > > Staring at the code....  I think you mean the opposite?  I notice
> > > do_sys_ftruncate() calling
> > > 
> > > 	do_truncate(dentry, length, ATTR_MTIME|ATTR_CTIME, file);
> > > 
> > > and do_sys_truncate() calling
> > > 
> > > 	do_truncate(path.dentry, length, 0, NULL);
> > > 
> > > where the third argument is getting OR'd with ATTR_FILE to pass into
> > > notify_change().
> > 
> > Sorry, yes. ftruncate() is the one that unconditionally sets the
> > mtime/ctime on success according to the POSIX spec.
> > 
> 
> Even when it's a noop? Blech.
> 
> > > Also even when a setattr does get through, I don't understand why it
> > should
> > > be invalidating our data cache.  Is there some reason it needs to, or
> > is this just
> > > a case that hasn't seemed worth fixing?
> > 
> > Is the problem perhaps that we should be clearing the
> > NFS_INO_INVALID_DATA flag in nfs_vmtruncate() when the size gets set to
> > zero?
> > 
> 
> That was my thinking too. Whenever we truncate the i_size to 0, we
> can safely assume that the pagecache is now valid, and should be able
> to clear NFS_INO_INVALID_DATA no matter when it was set, right?

I don't understand why 0 is a special case: why should my setting the
size ever mean that I have to go reread data from the server?

--b.