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

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

On Wed, Nov 02, 2011 at 07:07:42AM -0400, Jeff Layton wrote:
> On Tue, 1 Nov 2011 21:23:15 -0400
> "J. Bruce Fields" <bfields@fieldses.org> wrote:
> 
> > 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:
> > > > 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?
> > 
> 
> If the server doesn't send pre-op attrs (and linux servers don't) then
> you have no way to know whether someone raced in and did writes to the
> remaining pages just prior to your truncate.
> 
> Size 0 is a special case because there are no remaining pages.

Those 3rd-party writes could also arrive between the truncate and the
read of the post-op attrs.  Is this setattr/write conflict really any
different than write/write conflicts?  In both cases you've got a file
open for write on two different clients.

And we live with the same sort of race in the write/write case.  (Hence
the nfs_post_op_update_inode_force_wcc in nfs4_write_done_cb.)

--b.