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Bruce Fields" , Chuck Lever , Linux NFS Mailing List Subject: Re: [PATCH v5 4/5] NFSD: Return both a hole and a data segment Message-ID: <20200909202400.GB3894@fieldses.org> References: <20200908162559.509113-1-Anna.Schumaker@Netapp.com> <20200908162559.509113-5-Anna.Schumaker@Netapp.com> <20200908194944.GC6256@pick.fieldses.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) From: bfields@fieldses.org (J. Bruce Fields) Sender: linux-nfs-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-nfs@vger.kernel.org On Wed, Sep 09, 2020 at 12:51:38PM -0400, Anna Schumaker wrote: > On Tue, Sep 8, 2020 at 3:49 PM J. Bruce Fields wrote: > > > > On Tue, Sep 08, 2020 at 12:25:58PM -0400, schumaker.anna@gmail.com wrote: > > > From: Anna Schumaker > > > > > > But only one of each right now. We'll expand on this in the next patch. > > > > > > Signed-off-by: Anna Schumaker > > > --- > > > 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. Argh, right, I forgot the "if (nfserr && segments == 0)" at the end of nfsd4_encode_read_plus(). It's still possible to get a spurious error return if this happens at the very start of the READ_PLUS, though. Hm. Might be better to just encode another data segment? --b. > > 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 > > > > >