@@ -26,6 +26,40 @@
 #include "cache.h"
 #include "fid.h"
 
+/*
+ * Writeback calls this when it finds a folio that needs uploading.  This isn't
+ * called if writeback only has copy-to-cache to deal with.
+ */
+static void v9fs_begin_writeback(struct netfs_io_request *wreq)
+{
+	struct p9_fid *fid;
+
+	fid = v9fs_fid_find_inode(wreq->inode, true, INVALID_UID, true);
+	if (!fid) {
+		WARN_ONCE(1, "folio expected an open fid inode->i_ino=%lx\n",
+			  wreq->inode->i_ino);
+		return;
+	}
+
+	wreq->wsize = fid->clnt->msize - P9_IOHDRSZ;
+	if (fid->iounit)
+		wreq->wsize = min(wreq->wsize, fid->iounit);
+	wreq->netfs_priv = fid;
+	wreq->io_streams[0].avail = true;
+}
+
+/*
+ * Issue a subrequest to write to the server.
+ */
+static void v9fs_issue_write(struct netfs_io_subrequest *subreq)
+{
+	struct p9_fid *fid = subreq->rreq->netfs_priv;
+	int err, len;
+
+	len = p9_client_write(fid, subreq->start, &subreq->io_iter, &err);
+	netfs_write_subrequest_terminated(subreq, len ?: err, false);
+}
+
 static void v9fs_upload_to_server(struct netfs_io_subrequest *subreq)
 {
 	struct p9_fid *fid = subreq->rreq->netfs_priv;

@@ -92,6 +126,14 @@ static int v9fs_init_request(struct netfs_io_request *rreq, struct file *file)
 			rreq->origin == NETFS_UNBUFFERED_WRITE ||
 			rreq->origin == NETFS_DIO_WRITE);
 
+#if 0 // TODO: Cut over
+	if (rreq->origin == NETFS_WRITEBACK)
+		return 0; /* We don't get the write handle until we find we
+			   * have actually dirty data and not just
+			   * copy-to-cache data.
+			   */
+#endif
+
 	if (file) {
 		fid = file->private_data;
 		if (!fid)

@@ -103,6 +145,10 @@ static int v9fs_init_request(struct netfs_io_request *rreq, struct file *file)
 			goto no_fid;
 	}
 
+	rreq->wsize = fid->clnt->msize - P9_IOHDRSZ;
+	if (fid->iounit)
+		rreq->wsize = min(rreq->wsize, fid->iounit);
+
 	/* we might need to read from a fid that was opened write-only
 	 * for read-modify-write of page cache, use the writeback fid
 	 * for that */

@@ -131,6 +177,8 @@ const struct netfs_request_ops v9fs_req_ops = {
 	.init_request		= v9fs_init_request,
 	.free_request		= v9fs_free_request,
 	.issue_read		= v9fs_issue_read,
+	.begin_writeback	= v9fs_begin_writeback,
+	.issue_write		= v9fs_issue_write,
 	.create_write_requests	= v9fs_create_write_requests,
 };
 

@@ -207,6 +207,8 @@ int p9_client_read(struct p9_fid *fid, u64 offset, struct iov_iter *to, int *err
 int p9_client_read_once(struct p9_fid *fid, u64 offset, struct iov_iter *to,
 		int *err);
 int p9_client_write(struct p9_fid *fid, u64 offset, struct iov_iter *from, int *err);
+struct netfs_io_subrequest;
+void p9_client_write_subreq(struct netfs_io_subrequest *subreq);
 int p9_client_readdir(struct p9_fid *fid, char *data, u32 count, u64 offset);
 int p9dirent_read(struct p9_client *clnt, char *buf, int len,
 		  struct p9_dirent *dirent);

@@ -5,6 +5,7 @@
 
 menuconfig NET_9P
 	tristate "Plan 9 Resource Sharing Support (9P2000)"
+	select NETFS_SUPPORT
 	help
 	  If you say Y here, you will get experimental support for
 	  Plan 9 resource sharing via the 9P2000 protocol.

@@ -18,6 +18,7 @@
 #include <linux/sched/signal.h>
 #include <linux/uaccess.h>
 #include <linux/uio.h>
+#include <linux/netfs.h>
 #include <net/9p/9p.h>
 #include <linux/parser.h>
 #include <linux/seq_file.h>

@@ -1661,6 +1662,54 @@ p9_client_write(struct p9_fid *fid, u64 offset, struct iov_iter *from, int *err)
 }
 EXPORT_SYMBOL(p9_client_write);
 
+void
+p9_client_write_subreq(struct netfs_io_subrequest *subreq)
+{
+	struct netfs_io_request *wreq = subreq->rreq;
+	struct p9_fid *fid = wreq->netfs_priv;
+	struct p9_client *clnt = fid->clnt;
+	struct p9_req_t *req;
+	unsigned long long start = subreq->start + subreq->transferred;
+	size_t len = subreq->len - subreq->transferred;
+	int written, err;
+
+	p9_debug(P9_DEBUG_9P, ">>> TWRITE fid %d offset %llu len %zd\n",
+		 fid->fid, start, len);
+
+	/* Don't bother zerocopy for small IO (< 1024) */
+	if (clnt->trans_mod->zc_request && len > 1024) {
+		req = p9_client_zc_rpc(clnt, P9_TWRITE, NULL, &subreq->io_iter,
+				       0, wreq->len, P9_ZC_HDR_SZ, "dqd",
+				       fid->fid, start, len);
+	} else {
+		req = p9_client_rpc(clnt, P9_TWRITE, "dqV", fid->fid,
+				    start, len, &subreq->io_iter);
+	}
+	if (IS_ERR(req)) {
+		netfs_write_subrequest_terminated(subreq, PTR_ERR(req), false);
+		return;
+	}
+
+	err = p9pdu_readf(&req->rc, clnt->proto_version, "d", &written);
+	if (err) {
+		trace_9p_protocol_dump(clnt, &req->rc);
+		p9_req_put(clnt, req);
+		netfs_write_subrequest_terminated(subreq, err, false);
+		return;
+	}
+
+	if (written > len) {
+		pr_err("bogus RWRITE count (%d > %lu)\n", written, len);
+		written = len;
+	}
+
+	p9_debug(P9_DEBUG_9P, "<<< RWRITE count %zd\n", len);
+
+	p9_req_put(clnt, req);
+	netfs_write_subrequest_terminated(subreq, written, false);
+}
+EXPORT_SYMBOL(p9_client_write_subreq);
+
 struct p9_wstat *p9_client_stat(struct p9_fid *fid)
 {
 	int err;