Re: [PATCH] nfsd: map EBUSY for all operations

[Trond Myklebust <trondmy@hammerspace.com>]

On Mon, 2025-01-20 at 20:14 +0100, Amir Goldstein wrote:
> On Mon, Jan 20, 2025 at 7:45 PM Trond Myklebust
> <trondmy@hammerspace.com> wrote:
> > 
> > On Mon, 2025-01-20 at 19:21 +0100, Amir Goldstein wrote:
> > > On Mon, Jan 20, 2025 at 6:28 PM Trond Myklebust
> > > <trondmy@hammerspace.com> wrote:
> > > > 
> > > > On Mon, 2025-01-20 at 18:20 +0100, Amir Goldstein wrote:
> > > > > v4 client maps NFS4ERR_FILE_OPEN => EBUSY for all operations.
> > > > > 
> > > > > v4 server only maps EBUSY => NFS4ERR_FILE_OPEN for
> > > > > rmdir()/unlink()
> > > > > although it is also possible to get EBUSY from rename() for
> > > > > the
> > > > > same
> > > > > reason (victim is a local mount point).
> > > > > 
> > > > > Filesystems could return EBUSY for other operations, so just
> > > > > map
> > > > > it
> > > > > in server for all operations.
> > > > > 
> > > > > Signed-off-by: Amir Goldstein <amir73il@gmail.com>
> > > > > ---
> > > > > 
> > > > > Chuck,
> > > > > 
> > > > > I ran into this error with a FUSE filesystem and returns -
> > > > > EBUSY
> > > > > on
> > > > > open,
> > > > > but I noticed that vfs can also return EBUSY at least for
> > > > > rename().
> > > > > 
> > > > > Thanks,
> > > > > Amir.
> > > > > 
> > > > >  fs/nfsd/vfs.c | 10 ++--------
> > > > >  1 file changed, 2 insertions(+), 8 deletions(-)
> > > > > 
> > > > > diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
> > > > > index 29cb7b812d713..a61f99c081894 100644
> > > > > --- a/fs/nfsd/vfs.c
> > > > > +++ b/fs/nfsd/vfs.c
> > > > > @@ -100,6 +100,7 @@ nfserrno (int errno)
> > > > >               { nfserr_perm, -ENOKEY },
> > > > >               { nfserr_no_grace, -ENOGRACE},
> > > > >               { nfserr_io, -EBADMSG },
> > > > > +             { nfserr_file_open, -EBUSY},
> > > > >       };
> > > > >       int     i;
> > > > > 
> > > > > @@ -2006,14 +2007,7 @@ nfsd_unlink(struct svc_rqst *rqstp,
> > > > > struct
> > > > > svc_fh *fhp, int type,
> > > > >  out_drop_write:
> > > > >       fh_drop_write(fhp);
> > > > >  out_nfserr:
> > > > > -     if (host_err == -EBUSY) {
> > > > > -             /* name is mounted-on. There is no perfect
> > > > > -              * error status.
> > > > > -              */
> > > > > -             err = nfserr_file_open;
> > > > > -     } else {
> > > > > -             err = nfserrno(host_err);
> > > > > -     }
> > > > > +     err = nfserrno(host_err);
> > > > >  out:
> > > > >       return err;
> > > > >  out_unlock:
> > > > 
> > > > If this is a transient error, then it would seem that
> > > > NFS4ERR_DELAY
> > > > would be more appropriate.
> > > 
> > > It is not a transient error, not in the case of a fuse file open
> > > (it is busy as in locked for as long as it is going to be locked)
> > > and not in the case of failure to unlink/rename a local
> > > mountpoint.
> > > NFS4ERR_DELAY will cause the client to retry for a long time?
> > > 
> > > > NFS4ERR_FILE_OPEN is not supposed to apply
> > > > to directories, and so clients would be very confused about how
> > > > to
> > > > recover if you were to return it in this situation.
> > > 
> > > Do you mean specifically for OPEN command, because commit
> > > 466e16f0920f3 ("nfsd: check for EBUSY from vfs_rmdir/vfs_unink.")
> > > added the NFS4ERR_FILE_OPEN response for directories five years
> > > ago and vfs_rmdir can certainly return a non-transient EBUSY.
> > > 
> > 
> > I'm saying that clients expect NFS4ERR_FILE_OPEN to be returned in
> > response to LINK, REMOVE or RENAME only in situations where the
> > error
> > itself applies to a regular file.
> 
> This is very far from what upstream nfsd code implements (since 2019)
> 1. out of the above, only REMOVE returns NFS4ERR_FILE_OPEN
> 2. NFS4ERR_FILE_OPEN is not limited to non-dir
> 3. NFS4ERR_FILE_OPEN is not limited to silly renamed file -
>     it will also be the response for trying to rmdir a mount point
>     or trying to unlink a file which is a bind mount point

Fair enough. I believe the name given to this kind of server behaviour
is "bug".

> 
> > The protocol says that the client can expect this return value to
> > mean
> > it is dealing with a server with Windows-like semantics that
> > doesn't
> > allow these particular operations while the file is being held
> > open. It
> > says nothing about expecting the same behaviour for mountpoints,
> > and
> > since the latter have a very different life cycle than file open
> > state
> > does, you should not treat those cases as being the same.
> 
> The two cases are currently indistinguishable in nfsd_unlink(), but
> it could check DCACHE_NFSFS_RENAMED flag if we want to
> limit NFS4ERR_FILE_OPEN to this specific case - again, this is
> upstream code - nothing to do with my patch.
> 
> FWIW, my observed behavior of Linux nfs client for this error
> is about 1 second retries and failure with -EBUSY, which is fine
> for my use case, but if you think there is a better error to map
> EBUSY it's fine with me. nfsv3 maps it to EACCES anyway.
> 
> 

When doing LINK, RENAME, REMOVE on a mount point, I'd suggest returning
NFS4ERR_XDEV, since that is literally a case of trying to perform the
operation across a filesystem boundary.

Otherwise, since Linux doesn't implement Windows behaviour w.r.t. link,
rename or remove, it would seem that NFS4ERR_ACCESS is indeed the most
appropriate error, no? It's certainly the right behaviour for
sillyrenamed files.

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