Re: [PATCH v5 2/5] NFSD: Add READ_PLUS data support

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

On Wed, Sep 09, 2020 at 04:25:34PM -0400, bfields wrote:
> On Wed, Sep 09, 2020 at 12:53:18PM -0400, Anna Schumaker wrote:
> > On Tue, Sep 8, 2020 at 3:42 PM J. Bruce Fields <bfields@redhat.com> wrote:
> > >
> > > On Tue, Sep 08, 2020 at 12:25:56PM -0400, schumaker.anna@gmail.com wrote:
> > > > From: Anna Schumaker <Anna.Schumaker@Netapp.com>
> > > >
> > > > This patch adds READ_PLUS support for returning a single
> > > > NFS4_CONTENT_DATA segment to the client. This is basically the same as
> > > > the READ operation, only with the extra information about data segments.
> > > >
> > > > Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
> > > >
> > > > ---
> > > > v5: Fix up nfsd4_read_plus_rsize() calculation
> > > > ---
> > > >  fs/nfsd/nfs4proc.c | 20 +++++++++++
> > > >  fs/nfsd/nfs4xdr.c  | 83 ++++++++++++++++++++++++++++++++++++++++++++--
> > > >  2 files changed, 101 insertions(+), 2 deletions(-)
> > > >
> > > > diff --git a/fs/nfsd/nfs4proc.c b/fs/nfsd/nfs4proc.c
> > > > index eaf50eafa935..0a3df5f10501 100644
> > > > --- a/fs/nfsd/nfs4proc.c
> > > > +++ b/fs/nfsd/nfs4proc.c
> > > > @@ -2591,6 +2591,19 @@ static inline u32 nfsd4_read_rsize(struct svc_rqst *rqstp, struct nfsd4_op *op)
> > > >       return (op_encode_hdr_size + 2 + XDR_QUADLEN(rlen)) * sizeof(__be32);
> > > >  }
> > > >
> > > > +static inline u32 nfsd4_read_plus_rsize(struct svc_rqst *rqstp, struct nfsd4_op *op)
> > > > +{
> > > > +     u32 maxcount = svc_max_payload(rqstp);
> > > > +     u32 rlen = min(op->u.read.rd_length, maxcount);
> > > > +     /*
> > > > +      * Worst case is a single large data segment, so make
> > > > +      * sure we have enough room to encode that
> > > > +      */
> > >
> > > After the last patch we allow an unlimited number of segments.  So a
> > > zillion 1-byte segments is also possible, and is a worse case.
> > >
> > > Possible ways to avoid that kind of thing:
> > >
> > >         - when encoding, stop and return a short read if the xdr-encoded
> > >           result would exceed the limit calculated here.
> > 
> > Doesn't this happen automatically through calls to xdr_reserve_space()?
> 
> No, xdr_reserve_space() will keep us from running out of buffer
> completely, but it won't check that ops come in under the estimates made
> in read_plus_rsize().

If it's easier, another option might be just: if we ever get a "small"
hole (say, less than 512 bytes), just give up and encode the rest of the
result as a single big data segment.

--b.