[PATCH v3 6/6] NFSD: Repeal and replace the READ_PLUS implementation

[Anna Schumaker <anna@kernel.org>]

From: Anna Schumaker <Anna.Schumaker@Netapp.com>

Rather than relying on the underlying filesystem to tell us where hole
and data segments are through vfs_llseek(), let's instead do the hole
compression ourselves. This has a few advantages over the old
implementation:

1) A single call to the underlying filesystem through nfsd_readv() means
   the file can't change from underneath us in the middle of encoding.
2) A single call to the underlying filestem also means that the
   underlying filesystem only needs to synchronize cached and on-disk
   data one time instead of potentially many speeding up the reply.
3) Hole support for filesystems that don't support SEEK_HOLE and SEEK_DATA

I also included an optimization where we can cut down on the amount of
memory being shifed around by doing the compression as (hole, data)
pairs.

Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
---
 fs/nfsd/nfs4xdr.c | 219 +++++++++++++++++++++++++---------------------
 1 file changed, 119 insertions(+), 100 deletions(-)

diff --git a/fs/nfsd/nfs4xdr.c b/fs/nfsd/nfs4xdr.c
index 61b2aae81abb..df8289fce4ef 100644
--- a/fs/nfsd/nfs4xdr.c
+++ b/fs/nfsd/nfs4xdr.c
@@ -4731,81 +4731,138 @@ nfsd4_encode_offload_status(struct nfsd4_compoundres *resp, __be32 nfserr,
 	return nfserr;
 }
 
+struct read_plus_segment {
+	enum data_content4	rp_type;
+	u64			rp_offset;
+	u64			rp_length;
+	unsigned int		rp_page_pos;
+};
+
 static __be32
-nfsd4_encode_read_plus_data(struct nfsd4_compoundres *resp,
-			    struct nfsd4_read *read,
-			    unsigned long *maxcount, u32 *eof,
-			    loff_t *pos)
+nfsd4_read_plus_readv(struct nfsd4_compoundres *resp, struct nfsd4_read *read,
+		      unsigned long *maxcount, u32 *eof)
 {
 	struct xdr_stream *xdr = resp->xdr;
-	struct file *file = read->rd_nf->nf_file;
-	int starting_len = xdr->buf->len;
-	loff_t hole_pos;
-	__be32 nfserr;
-	__be32 *p, tmp;
-	__be64 tmp64;
-
-	hole_pos = pos ? *pos : vfs_llseek(file, read->rd_offset, SEEK_HOLE);
-	if (hole_pos > read->rd_offset)
-		*maxcount = min_t(unsigned long, *maxcount, hole_pos - read->rd_offset);
-	*maxcount = min_t(unsigned long, *maxcount, (xdr->buf->buflen - xdr->buf->len));
-
-	/* Content type, offset, byte count */
-	p = xdr_reserve_space(xdr, 4 + 8 + 4);
-	if (!p)
-		return nfserr_resource;
+	unsigned int starting_len = xdr->buf->len;
+	__be32 nfserr, zero = xdr_zero;
+	unsigned int pad;
 
+	/*
+	 * Reserve the maximum abount of space needed to craft a READ_PLUS
+	 * reply. The call to xdr_reserve_space_vec() switches us to the
+	 * xdr->pages, which we then read file data into before analyzing
+	 * the individual segments.
+	 */
 	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);
+	nfserr = nfsd_readv(resp->rqstp, read->rd_fhp, read->rd_nf->nf_file,
+			    read->rd_offset, resp->rqstp->rq_vec, read->rd_vlen,
+			    maxcount, eof);
 	if (nfserr)
 		return nfserr;
-	xdr_truncate_encode(xdr, starting_len + 16 + xdr_align_size(*maxcount));
+	xdr_truncate_encode(xdr, starting_len + xdr_align_size(*maxcount));
 
-	tmp = htonl(NFS4_CONTENT_DATA);
-	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);
-	write_bytes_to_xdr_buf(xdr->buf, starting_len + 12, &tmp,   4);
-
-	tmp = xdr_zero;
-	write_bytes_to_xdr_buf(xdr->buf, starting_len + 16 + *maxcount, &tmp,
-			       xdr_pad_size(*maxcount));
+	pad = xdr_pad_size(*maxcount);
+	write_bytes_to_xdr_buf(xdr->buf, starting_len + *maxcount, &zero, pad);
 	return nfs_ok;
 }
 
+/**
+ * nfsd4_encode_read_plus_segment - Encode a single READ_PLUS segment
+ * @xdr: pointer to an xdr_stream
+ * @segment: pointer to a single segment
+ * @bufpos: xdr_stream offset to place the segment
+ * @segments: pointer to the total number of segments seen
+ *
+ * Performs surgery on the xdr_stream to compress out HOLE segments and
+ * to place DATA segments in the proper place.
+ */
+static void
+nfsd4_encode_read_plus_segment(struct xdr_stream *xdr,
+			       struct read_plus_segment *segment,
+			       unsigned int *bufpos, unsigned int *segments)
+{
+	struct xdr_buf *buf = xdr->buf;
+
+	xdr_encode_word(buf, *bufpos, segment->rp_type);
+	xdr_encode_double(buf, *bufpos + XDR_UNIT, segment->rp_offset);
+	*bufpos += 3 * XDR_UNIT;
+
+	if (segment->rp_type == NFS4_CONTENT_HOLE) {
+		xdr_encode_double(buf, *bufpos, segment->rp_length);
+		*bufpos += 2 * XDR_UNIT;
+	} else {
+		size_t align = xdr_align_size(segment->rp_length);
+		xdr_encode_word(buf, *bufpos, segment->rp_length);
+		if (*segments == 0)
+			xdr_buf_trim_head(buf, XDR_UNIT);
+
+		xdr_stream_move_subsegment(xdr,
+				buf->head[0].iov_len + segment->rp_page_pos,
+				*bufpos + XDR_UNIT, align);
+		*bufpos += XDR_UNIT + align;
+	}
+
+	*segments += 1;
+}
+
 static __be32
-nfsd4_encode_read_plus_hole(struct nfsd4_compoundres *resp,
-			    struct nfsd4_read *read,
-			    unsigned long *maxcount, u32 *eof)
+nfsd4_encode_read_plus_segments(struct nfsd4_compoundres *resp,
+				struct nfsd4_read *read,
+				unsigned int *segments, u32 *eof)
 {
-	struct file *file = read->rd_nf->nf_file;
-	loff_t data_pos = vfs_llseek(file, read->rd_offset, SEEK_DATA);
-	loff_t f_size = i_size_read(file_inode(file));
-	unsigned long count;
-	__be32 *p;
+	struct xdr_stream *xdr = resp->xdr;
+	unsigned int bufpos = xdr->buf->len;
+	u64 offset = read->rd_offset;
+	struct read_plus_segment segment;
+	enum data_content4 pagetype;
+	unsigned long maxcount;
+	unsigned int pagenum = 0;
+	unsigned int pagelen;
+	char *vpage, *p;
+	__be32 nfserr;
 
-	if (data_pos == -ENXIO)
-		data_pos = f_size;
-	else if (data_pos <= read->rd_offset || (data_pos < f_size && data_pos % PAGE_SIZE))
-		return nfsd4_encode_read_plus_data(resp, read, maxcount, eof, &f_size);
-	count = data_pos - read->rd_offset;
-
-	/* Content type, offset, byte count */
-	p = xdr_reserve_space(resp->xdr, 4 + 8 + 8);
-	if (!p)
+	/* enough space for a HOLE segment before we switch to the pages */
+	if (!xdr_reserve_space(xdr, 5 * XDR_UNIT))
 		return nfserr_resource;
+	xdr_commit_encode(xdr);
 
-	*p++ = htonl(NFS4_CONTENT_HOLE);
-	p = xdr_encode_hyper(p, read->rd_offset);
-	p = xdr_encode_hyper(p, count);
+	maxcount = min_t(unsigned long, read->rd_length,
+			 (xdr->buf->buflen - xdr->buf->len));
 
-	*eof = (read->rd_offset + count) >= f_size;
-	*maxcount = min_t(unsigned long, count, *maxcount);
+	nfserr = nfsd4_read_plus_readv(resp, read, &maxcount, eof);
+	if (nfserr)
+		return nfserr;
+
+	while (maxcount > 0) {
+		vpage = xdr_buf_nth_page_address(xdr->buf, pagenum, &pagelen);
+		pagelen = min_t(unsigned int, pagelen, maxcount);
+		if (!vpage || pagelen == 0)
+			break;
+		p = memchr_inv(vpage, 0, pagelen);
+		pagetype = (p == NULL) ? NFS4_CONTENT_HOLE : NFS4_CONTENT_DATA;
+
+		if (pagetype != segment.rp_type || pagenum == 0) {
+			if (likely(pagenum > 0)) {
+				nfsd4_encode_read_plus_segment(xdr, &segment,
+							      &bufpos, segments);
+				offset += segment.rp_length;
+			}
+			segment.rp_type = pagetype;
+			segment.rp_offset = offset;
+			segment.rp_length = pagelen;
+			segment.rp_page_pos = pagenum * PAGE_SIZE;
+		} else
+			segment.rp_length += pagelen;
+
+		maxcount -= pagelen;
+		pagenum++;
+	}
+
+	nfsd4_encode_read_plus_segment(xdr, &segment, &bufpos, segments);
+	xdr_truncate_encode(xdr, bufpos);
 	return nfs_ok;
 }
 
@@ -4813,69 +4870,31 @@ static __be32
 nfsd4_encode_read_plus(struct nfsd4_compoundres *resp, __be32 nfserr,
 		       struct nfsd4_read *read)
 {
-	unsigned long maxcount, count;
 	struct xdr_stream *xdr = resp->xdr;
-	struct file *file;
 	int starting_len = xdr->buf->len;
-	int last_segment = xdr->buf->len;
-	int segments = 0;
-	__be32 *p, tmp;
-	bool is_data;
-	loff_t pos;
+	unsigned int segments = 0;
 	u32 eof;
 
 	if (nfserr)
 		return nfserr;
-	file = read->rd_nf->nf_file;
 
 	/* eof flag, segment count */
-	p = xdr_reserve_space(xdr, 4 + 4);
-	if (!p)
+	if (!xdr_reserve_space(xdr, 2 * XDR_UNIT))
 		return nfserr_resource;
 	xdr_commit_encode(xdr);
 
-	maxcount = min_t(unsigned long, read->rd_length,
-			 (xdr->buf->buflen - xdr->buf->len));
-	count    = maxcount;
-
-	eof = read->rd_offset >= i_size_read(file_inode(file));
+	eof = read->rd_offset >= i_size_read(file_inode(read->rd_nf->nf_file));
 	if (eof)
 		goto out;
 
-	pos = vfs_llseek(file, read->rd_offset, SEEK_HOLE);
-	is_data = pos > read->rd_offset;
-
-	while (count > 0 && !eof) {
-		maxcount = count;
-		if (is_data)
-			nfserr = nfsd4_encode_read_plus_data(resp, read, &maxcount, &eof,
-						segments == 0 ? &pos : NULL);
-		else
-			nfserr = nfsd4_encode_read_plus_hole(resp, read, &maxcount, &eof);
-		if (nfserr)
-			goto out;
-		count -= maxcount;
-		read->rd_offset += maxcount;
-		is_data = !is_data;
-		last_segment = xdr->buf->len;
-		segments++;
-	}
-
+	nfserr = nfsd4_encode_read_plus_segments(resp, read, &segments, &eof);
 out:
-	if (nfserr && segments == 0)
+	if (nfserr)
 		xdr_truncate_encode(xdr, starting_len);
 	else {
-		if (nfserr) {
-			xdr_truncate_encode(xdr, last_segment);
-			nfserr = nfs_ok;
-			eof = 0;
-		}
-		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);
+		xdr_encode_word(xdr->buf, starting_len, eof);
+		xdr_encode_word(xdr->buf, starting_len + XDR_UNIT, segments);
 	}
-
 	return nfserr;
 }
 
-- 
2.37.1