Re: [PATCH 2/2] NFSD: allow client to use write delegation stateid for READ

[Jeff Layton <jlayton@kernel.org>]

On Thu, 2025-02-13 at 16:07 -0500, Jeff Layton wrote:
> On Thu, 2025-02-13 at 11:37 -0800, Dai Ngo wrote:
> > Allow read using write delegation stateid granted on OPENs with
> > OPEN4_SHARE_ACCESS_WRITE only, to accommodate clients whose WRITE
> > implementation may unavoidably do (e.g., due to buffer cache
> > constraints).
> > 
> > When this condition is detected in nfsd4_encode_read the access
> > mode FMODE_READ is temporarily added to the file's f_mode and is
> > removed when the read is done.
> > 
> > Signed-off-by: Dai Ngo <dai.ngo@oracle.com>
> > ---
> >  fs/nfsd/nfs4proc.c | 15 ++++++++++++++-
> >  fs/nfsd/nfs4xdr.c  |  8 ++++++++
> >  fs/nfsd/xdr4.h     |  1 +
> >  3 files changed, 23 insertions(+), 1 deletion(-)
> > 
> > diff --git a/fs/nfsd/nfs4proc.c b/fs/nfsd/nfs4proc.c
> > index f6e06c779d09..be43627bbf78 100644
> > --- a/fs/nfsd/nfs4proc.c
> > +++ b/fs/nfsd/nfs4proc.c
> > @@ -973,7 +973,18 @@ nfsd4_read(struct svc_rqst *rqstp, struct nfsd4_compound_state *cstate,
> >  	/* check stateid */
> >  	status = nfs4_preprocess_stateid_op(rqstp, cstate, &cstate->current_fh,
> >  					&read->rd_stateid, RD_STATE,
> > -					&read->rd_nf, NULL);
> > +					&read->rd_nf, &read->rd_wd_stid);
> > +	/*
> > +	 * rd_wd_stid is needed for nfsd4_encode_read to allow write
> > +	 * delegation stateid used for read. Its refcount is decremented
> > +	 * by nfsd4_read_release when read is done.
> > +	 */
> > +	if (!status && read->rd_wd_stid &&
> > +		(read->rd_wd_stid->sc_type != SC_TYPE_DELEG ||
> > +		delegstateid(read->rd_wd_stid)->dl_type != NFS4_OPEN_DELEGATE_WRITE)) {
> > +		nfs4_put_stid(read->rd_wd_stid);
> > +		read->rd_wd_stid = NULL;
> > +	}
> >  
> >  	read->rd_rqstp = rqstp;
> >  	read->rd_fhp = &cstate->current_fh;
> > @@ -984,6 +995,8 @@ nfsd4_read(struct svc_rqst *rqstp, struct nfsd4_compound_state *cstate,
> >  static void
> >  nfsd4_read_release(union nfsd4_op_u *u)
> >  {
> > +	if (u->read.rd_wd_stid)
> > +		nfs4_put_stid(u->read.rd_wd_stid);
> >  	if (u->read.rd_nf)
> >  		nfsd_file_put(u->read.rd_nf);
> >  	trace_nfsd_read_done(u->read.rd_rqstp, u->read.rd_fhp,
> > diff --git a/fs/nfsd/nfs4xdr.c b/fs/nfsd/nfs4xdr.c
> > index e67420729ecd..3996678bab3f 100644
> > --- a/fs/nfsd/nfs4xdr.c
> > +++ b/fs/nfsd/nfs4xdr.c
> > @@ -4498,6 +4498,7 @@ nfsd4_encode_read(struct nfsd4_compoundres *resp, __be32 nfserr,
> >  	unsigned long maxcount;
> >  	__be32 wire_data[2];
> >  	struct file *file;
> > +	bool wronly = false;
> >  
> >  	if (nfserr)
> >  		return nfserr;
> > @@ -4515,10 +4516,17 @@ nfsd4_encode_read(struct nfsd4_compoundres *resp, __be32 nfserr,
> >  	maxcount = min_t(unsigned long, read->rd_length,
> >  			 (xdr->buf->buflen - xdr->buf->len));
> >  
> > +	if (!(file->f_mode & FMODE_READ) && read->rd_wd_stid) {
> > +		/* allow READ using write delegation stateid */
> > +		wronly = true;
> > +		file->f_mode |= FMODE_READ;
> > +	}
> 
> Is that really OK? Can we just upgrade the f_mode like that?
> 
> Also, what happens with more exotic exported filesystems like NFS? 
> 
> For example, if I'm reexporting NFS, the backend NFS server may not
> allow you to do a READ operation using a OPEN4_SHARE_ACCESS_WRITE only
> stateid. Won't this break in that case?
> 

Hmm...bad example since we don't allow delegations on reexported NFS
these days. Reexporting Ceph or SMB might be a better example. They'll
likely both have problems if you try to issue a read on the result from
a O_WRONLY open. I think you will probably need to rework the way
nfs4_file's track their struct files.

IOW, when the client does a OPEN4_SHARE_ACCESS_WRITE-only open, you
need to get a struct file that is FMODE_READ|FMODE_WRITE to hang off
the delegation. But, you'll also need to fix up the accounting for the
share/deny mode locking to ignore that you _actually_ have it open for
read too in that case.

Smoke and mirrors...

> >  	if (file->f_op->splice_read && splice_ok)
> >  		nfserr = nfsd4_encode_splice_read(resp, read, file, maxcount);
> >  	else
> >  		nfserr = nfsd4_encode_readv(resp, read, file, maxcount);
> > +	if (wronly)
> > +		file->f_mode &= ~FMODE_READ;
> >  	if (nfserr) {
> >  		xdr_truncate_encode(xdr, eof_offset);
> >  		return nfserr;
> > diff --git a/fs/nfsd/xdr4.h b/fs/nfsd/xdr4.h
> > index c26ba86dbdfd..2f053beed899 100644
> > --- a/fs/nfsd/xdr4.h
> > +++ b/fs/nfsd/xdr4.h
> > @@ -426,6 +426,7 @@ struct nfsd4_read {
> >  	struct svc_rqst		*rd_rqstp;          /* response */
> >  	struct svc_fh		*rd_fhp;            /* response */
> >  	u32			rd_eof;             /* response */
> > +	struct nfs4_stid	*rd_wd_stid;        /* internal */
> >  };
> >  
> >  struct nfsd4_readdir {
> 

-- 
Jeff Layton <jlayton@kernel.org>