On Tue, Sep 8, 2020 at 3:49 PM J. Bruce Fields <bfields@xxxxxxxxxx> wrote:
>
> On Tue, Sep 08, 2020 at 12:25:58PM -0400, schumaker.anna@xxxxxxxxx wrote:
> > From: Anna Schumaker <Anna.Schumaker@xxxxxxxxxx>
> >
> > But only one of each right now. We'll expand on this in the next patch.
> >
> > Signed-off-by: Anna Schumaker <Anna.Schumaker@xxxxxxxxxx>
> > ---
> > 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
> >
>
