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References: <20200908162559.509113-1-Anna.Schumaker@Netapp.com>
 <20200908162559.509113-5-Anna.Schumaker@Netapp.com> <20200908194944.GC6256@pick.fieldses.org>
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From:   Anna Schumaker <schumaker.anna@gmail.com>
Date:   Wed, 9 Sep 2020 12:51:38 -0400
Message-ID: <CAFX2Jfn7Fb=e2Sigf0xEZ4tw5h0KMnyOQWi5MCvdfq+GFXj+-A@mail.gmail.com>
Subject: Re: [PATCH v5 4/5] NFSD: Return both a hole and a data segment
To:     "J. Bruce Fields" <bfields@redhat.com>
Cc:     Chuck Lever <chuck.lever@oracle.com>,
        Linux NFS Mailing List <linux-nfs@vger.kernel.org>
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On Tue, Sep 8, 2020 at 3:49 PM J. Bruce Fields <bfields@redhat.com> wrote:
>
> On Tue, Sep 08, 2020 at 12:25:58PM -0400, schumaker.anna@gmail.com wrote:
> > From: Anna Schumaker <Anna.Schumaker@Netapp.com>
> >
> > But only one of each right now. We'll expand on this in the next patch.
> >
> > Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
> > ---
> > v5: If we've already encoded a segment, then return a short read if
> >     later segments return an error for some reason.
> > ---
> >  fs/nfsd/nfs4xdr.c | 54 ++++++++++++++++++++++++++++++++++-------------
> >  1 file changed, 39 insertions(+), 15 deletions(-)
> >
> > diff --git a/fs/nfsd/nfs4xdr.c b/fs/nfsd/nfs4xdr.c
> > index 45159bd9e9a4..856606263c1d 100644
> > --- a/fs/nfsd/nfs4xdr.c
> > +++ b/fs/nfsd/nfs4xdr.c
> > @@ -4603,7 +4603,7 @@ nfsd4_encode_offload_status(struct nfsd4_compoundres *resp, __be32 nfserr,
> >  static __be32
> >  nfsd4_encode_read_plus_data(struct nfsd4_compoundres *resp,
> >                           struct nfsd4_read *read,
> > -                         unsigned long maxcount,  u32 *eof)
> > +                         unsigned long *maxcount, u32 *eof)
> >  {
> >       struct xdr_stream *xdr = &resp->xdr;
> >       struct file *file = read->rd_nf->nf_file;
> > @@ -4614,19 +4614,19 @@ nfsd4_encode_read_plus_data(struct nfsd4_compoundres *resp,
> >       __be64 tmp64;
> >
> >       if (hole_pos > read->rd_offset)
> > -             maxcount = min_t(unsigned long, maxcount, hole_pos - read->rd_offset);
> > +             *maxcount = min_t(unsigned long, *maxcount, hole_pos - read->rd_offset);
> >
> >       /* Content type, offset, byte count */
> >       p = xdr_reserve_space(xdr, 4 + 8 + 4);
> >       if (!p)
> >               return nfserr_resource;
> >
> > -     read->rd_vlen = xdr_reserve_space_vec(xdr, resp->rqstp->rq_vec, maxcount);
> > +     read->rd_vlen = xdr_reserve_space_vec(xdr, resp->rqstp->rq_vec, *maxcount);
> >       if (read->rd_vlen < 0)
> >               return nfserr_resource;
> >
> >       nfserr = nfsd_readv(resp->rqstp, read->rd_fhp, file, read->rd_offset,
> > -                         resp->rqstp->rq_vec, read->rd_vlen, &maxcount, eof);
> > +                         resp->rqstp->rq_vec, read->rd_vlen, maxcount, eof);
> >       if (nfserr)
> >               return nfserr;
> >
> > @@ -4634,7 +4634,7 @@ nfsd4_encode_read_plus_data(struct nfsd4_compoundres *resp,
> >       write_bytes_to_xdr_buf(xdr->buf, starting_len,      &tmp,   4);
> >       tmp64 = cpu_to_be64(read->rd_offset);
> >       write_bytes_to_xdr_buf(xdr->buf, starting_len + 4,  &tmp64, 8);
> > -     tmp = htonl(maxcount);
> > +     tmp = htonl(*maxcount);
> >       write_bytes_to_xdr_buf(xdr->buf, starting_len + 12, &tmp,   4);
> >       return nfs_ok;
> >  }
> > @@ -4642,11 +4642,19 @@ nfsd4_encode_read_plus_data(struct nfsd4_compoundres *resp,
> >  static __be32
> >  nfsd4_encode_read_plus_hole(struct nfsd4_compoundres *resp,
> >                           struct nfsd4_read *read,
> > -                         unsigned long maxcount, u32 *eof)
> > +                         unsigned long *maxcount, u32 *eof)
> >  {
> >       struct file *file = read->rd_nf->nf_file;
> > +     loff_t data_pos = vfs_llseek(file, read->rd_offset, SEEK_DATA);
> > +     unsigned long count;
> >       __be32 *p;
> >
> > +     if (data_pos == -ENXIO)
> > +             data_pos = i_size_read(file_inode(file));
> > +     else if (data_pos <= read->rd_offset)
> > +             return nfserr_resource;
>
> I think there's still a race here:
>
>         vfs_llseek(.,0,SEEK_HOLE) returns 1024
>         read 1024 bytes of data
>                                         another process fills the hole
>         vfs_llseek(.,1024,SEEK_DATA) returns 1024
>         code above returns nfserr_resource
>
> We end up returning an error to the client when we should have just
> returned more data.

As long as we've encoded at least one segment successfully, we'll
actually return a short read in this case (as of the most recent
patches). I tried implementing a check for what each segment actually
was before encoding, but it lead to a lot of extra lseeks (so
potential for races / performance problems on btrfs). Returning a
short read seemed like a better approach to me.

Anna

>
> --b.
>
> > +     count = data_pos - read->rd_offset;
> > +
> >       /* Content type, offset, byte count */
> >       p = xdr_reserve_space(&resp->xdr, 4 + 8 + 8);
> >       if (!p)
> > @@ -4654,9 +4662,10 @@ nfsd4_encode_read_plus_hole(struct nfsd4_compoundres *resp,
> >
> >       *p++ = htonl(NFS4_CONTENT_HOLE);
> >        p   = xdr_encode_hyper(p, read->rd_offset);
> > -      p   = xdr_encode_hyper(p, maxcount);
> > +      p   = xdr_encode_hyper(p, count);
> >
> > -     *eof = (read->rd_offset + maxcount) >= i_size_read(file_inode(file));
> > +     *eof = (read->rd_offset + count) >= i_size_read(file_inode(file));
> > +     *maxcount = min_t(unsigned long, count, *maxcount);
> >       return nfs_ok;
> >  }
> >
> > @@ -4664,7 +4673,7 @@ static __be32
> >  nfsd4_encode_read_plus(struct nfsd4_compoundres *resp, __be32 nfserr,
> >                      struct nfsd4_read *read)
> >  {
> > -     unsigned long maxcount;
> > +     unsigned long maxcount, count;
> >       struct xdr_stream *xdr = &resp->xdr;
> >       struct file *file;
> >       int starting_len = xdr->buf->len;
> > @@ -4687,6 +4696,7 @@ nfsd4_encode_read_plus(struct nfsd4_compoundres *resp, __be32 nfserr,
> >       maxcount = min_t(unsigned long, maxcount,
> >                        (xdr->buf->buflen - xdr->buf->len));
> >       maxcount = min_t(unsigned long, maxcount, read->rd_length);
> > +     count    = maxcount;
> >
> >       eof = read->rd_offset >= i_size_read(file_inode(file));
> >       if (eof)
> > @@ -4695,24 +4705,38 @@ nfsd4_encode_read_plus(struct nfsd4_compoundres *resp, __be32 nfserr,
> >       pos = vfs_llseek(file, read->rd_offset, SEEK_DATA);
> >       if (pos == -ENXIO)
> >               pos = i_size_read(file_inode(file));
> > +     else if (pos < 0)
> > +             pos = read->rd_offset;
> >
> > -     if (pos > read->rd_offset) {
> > -             maxcount = pos - read->rd_offset;
> > -             nfserr = nfsd4_encode_read_plus_hole(resp, read, maxcount, &eof);
> > +     if (pos == read->rd_offset) {
> > +             maxcount = count;
> > +             nfserr = nfsd4_encode_read_plus_data(resp, read, &maxcount, &eof);
> > +             if (nfserr)
> > +                     goto out;
> > +             count -= maxcount;
> > +             read->rd_offset += maxcount;
> >               segments++;
> > -     } else {
> > -             nfserr = nfsd4_encode_read_plus_data(resp, read, maxcount, &eof);
> > +     }
> > +
> > +     if (count > 0 && !eof) {
> > +             maxcount = count;
> > +             nfserr = nfsd4_encode_read_plus_hole(resp, read, &maxcount, &eof);
> > +             if (nfserr)
> > +                     goto out;
> > +             count -= maxcount;
> > +             read->rd_offset += maxcount;
> >               segments++;
> >       }
> >
> >  out:
> > -     if (nfserr)
> > +     if (nfserr && segments == 0)
> >               xdr_truncate_encode(xdr, starting_len);
> >       else {
> >               tmp = htonl(eof);
> >               write_bytes_to_xdr_buf(xdr->buf, starting_len,     &tmp, 4);
> >               tmp = htonl(segments);
> >               write_bytes_to_xdr_buf(xdr->buf, starting_len + 4, &tmp, 4);
> > +             nfserr = nfs_ok;
> >       }
> >
> >       return nfserr;
> > --
> > 2.28.0
> >
>