[Linux-cachefs] Adventures in NFS re-exporting

[J. Bruce Fields]

On Wed, Sep 30, 2020 at 03:30:22PM -0400, Jeff Layton wrote:
> On Tue, 2020-09-22 at 13:31 +0100, Daire Byrne wrote:
> > This patch helps to avoid this when applied to the re-export server but there may be other places where this happens too. I accept that this patch is probably not the right/general way to do this, but it helps to highlight the issue when re-exporting and it works well for our use case:
> > 
> > --- linux-5.5.0-1.el7.x86_64/fs/nfs/inode.c     2020-01-27 00:23:03.000000000 +0000
> > +++ new/fs/nfs/inode.c  2020-02-13 16:32:09.013055074 +0000
> > @@ -1869,7 +1869,7 @@
> >  
> >         /* More cache consistency checks */
> >         if (fattr->valid & NFS_ATTR_FATTR_CHANGE) {
> > -               if (!inode_eq_iversion_raw(inode, fattr->change_attr)) {
> > +               if (inode_peek_iversion_raw(inode) < fattr->change_attr) {
> >                         /* Could it be a race with writeback? */
> >                         if (!(have_writers || have_delegation)) {
> >                                 invalid |= NFS_INO_INVALID_DATA
> > 
> > With this patch, the re-export server's NFS client attribute cache is maintained and used by all the clients that then mount it. When many hundreds of clients are all doing similar things at the same time, the re-export server's NFS client cache is invaluable in accelerating the lookups (getattrs).
> > 
> > Perhaps a more correct approach would be to detect when it is knfsd that is accessing the client mount and change the cache consistency checks accordingly? 
> 
> Yeah, I don't think you can do this for the reasons Trond outlined.

I'm not clear whether Trond thought that knfsd's behavior in the case it
returns NFS4_CHANGE_TYPE_IS_MONOTONIC_INCR might be good enough to allow
this or some other optimization.

--b.