Re: [PATCH v4 2/5] NFSD: Add READ_PLUS data support

From: bfields@fieldses.org (J. Bruce Fields)
To: Anna Schumaker <schumaker.anna@gmail.com>
Cc: "J. Bruce Fields" <bfields@redhat.com>,
	Chuck Lever <chuck.lever@oracle.com>,
	Linux NFS Mailing List <linux-nfs@vger.kernel.org>
Subject: Re: [PATCH v4 2/5] NFSD: Add READ_PLUS data support
Date: Tue, 1 Sep 2020 12:49:38 -0400	[thread overview]
Message-ID: <20200901164938.GC12082@fieldses.org> (raw)
In-Reply-To: <CAFX2Jfn3LN9Zc-=4mAm1mQ3k8PN6C1yF4xqh6B-yyXCxFnp7hQ@mail.gmail.com>

On Mon, Aug 31, 2020 at 02:16:26PM -0400, Anna Schumaker wrote:
> On Fri, Aug 28, 2020 at 5:56 PM J. Bruce Fields <bfields@redhat.com> wrote:
> >
> > On Fri, Aug 28, 2020 at 05:25:21PM -0400, J. Bruce Fields wrote:
> > > On Mon, Aug 17, 2020 at 12:53:07PM -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>
> > > > ---
> > > >  fs/nfsd/nfs4proc.c | 17 ++++++++++
> > > >  fs/nfsd/nfs4xdr.c  | 83 ++++++++++++++++++++++++++++++++++++++++++++--
> > > >  2 files changed, 98 insertions(+), 2 deletions(-)
> > > >
> > > > diff --git a/fs/nfsd/nfs4proc.c b/fs/nfsd/nfs4proc.c
> > > > index a09c35f0f6f0..9630d33211f2 100644
> > > > --- a/fs/nfsd/nfs4proc.c
> > > > +++ b/fs/nfsd/nfs4proc.c
> > > > @@ -2523,6 +2523,16 @@ 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);
> > > > +   /* enough extra xdr space for encoding either a hole or data segment. */
> > > > +   u32 segments = 1 + 2 + 2;
> > > > +
> > > > +   return (op_encode_hdr_size + 2 + segments + XDR_QUADLEN(rlen)) * sizeof(__be32);
> > >
> > > I'm not sure I understand this calculation.
> > >
> > > In the final code, there's no fixed limit on the number of segments
> > > returned by a single READ_PLUS op, right?
> >
> > I think the worst-case overhead to represent a hole is around 50 bytes.
> >
> > So as long as we don't encode any holes less than that, then we can just
> > use rlen as an upper bound.
> >
> > We really don't want to bother encoding small holes.  I doubt
> > filesystems want to bother with them either.  Do they give us any
> > guarantees as to the minimum size of a hole?
> 
> The minimum size seems to be PAGE_SIZE from everything I've seen.

OK, can we make that assumption explicit?  It'd simplify stuff like
this.

--b.