1 files changed, 330 insertions, 93 deletions

diff --git a/fs/netfs/write_issue.c b/fs/netfs/write_issue.c
index d7c971df8866..dd8743bc8d7f 100644
--- a/fs/netfs/write_issue.c
+++ b/fs/netfs/write_issue.c
@@ -94,39 +94,47 @@ struct netfs_io_request *netfs_create_write_req(struct address_space *mapping,
 {
 	struct netfs_io_request *wreq;
 	struct netfs_inode *ictx;
+	bool is_cacheable = (origin == NETFS_WRITEBACK ||
+			     origin == NETFS_WRITEBACK_SINGLE ||
+			     origin == NETFS_WRITETHROUGH ||
+			     origin == NETFS_PGPRIV2_COPY_TO_CACHE);
 
 	wreq = netfs_alloc_request(mapping, file, start, 0, origin);
 	if (IS_ERR(wreq))
 		return wreq;
 
-	kenter("R=%x", wreq->debug_id);
+	_enter("R=%x", wreq->debug_id);
 
 	ictx = netfs_inode(wreq->inode);
-	if (test_bit(NETFS_RREQ_WRITE_TO_CACHE, &wreq->flags))
+	if (is_cacheable && netfs_is_cache_enabled(ictx))
 		fscache_begin_write_operation(&wreq->cache_resources, netfs_i_cookie(ictx));
+	if (rolling_buffer_init(&wreq->buffer, wreq->debug_id, ITER_SOURCE) < 0)
+		goto nomem;
 
-	wreq->contiguity = wreq->start;
 	wreq->cleaned_to = wreq->start;
-	INIT_WORK(&wreq->work, netfs_write_collection_worker);
 
 	wreq->io_streams[0].stream_nr		= 0;
 	wreq->io_streams[0].source		= NETFS_UPLOAD_TO_SERVER;
 	wreq->io_streams[0].prepare_write	= ictx->ops->prepare_write;
 	wreq->io_streams[0].issue_write		= ictx->ops->issue_write;
 	wreq->io_streams[0].collected_to	= start;
-	wreq->io_streams[0].transferred		= LONG_MAX;
+	wreq->io_streams[0].transferred		= 0;
 
 	wreq->io_streams[1].stream_nr		= 1;
 	wreq->io_streams[1].source		= NETFS_WRITE_TO_CACHE;
 	wreq->io_streams[1].collected_to	= start;
-	wreq->io_streams[1].transferred		= LONG_MAX;
+	wreq->io_streams[1].transferred		= 0;
 	if (fscache_resources_valid(&wreq->cache_resources)) {
 		wreq->io_streams[1].avail	= true;
+		wreq->io_streams[1].active	= true;
 		wreq->io_streams[1].prepare_write = wreq->cache_resources.ops->prepare_write_subreq;
 		wreq->io_streams[1].issue_write = wreq->cache_resources.ops->issue_write;
 	}
 
 	return wreq;
+nomem:
+	netfs_put_failed_request(wreq);
+	return ERR_PTR(-ENOMEM);
 }
 
 /**
@@ -151,26 +159,32 @@ static void netfs_prepare_write(struct netfs_io_request *wreq,
 				loff_t start)
 {
 	struct netfs_io_subrequest *subreq;
+	struct iov_iter *wreq_iter = &wreq->buffer.iter;
+
+	/* Make sure we don't point the iterator at a used-up folio_queue
+	 * struct being used as a placeholder to prevent the queue from
+	 * collapsing.  In such a case, extend the queue.
+	 */
+	if (iov_iter_is_folioq(wreq_iter) &&
+	    wreq_iter->folioq_slot >= folioq_nr_slots(wreq_iter->folioq))
+		rolling_buffer_make_space(&wreq->buffer);
 
 	subreq = netfs_alloc_subrequest(wreq);
 	subreq->source		= stream->source;
 	subreq->start		= start;
-	subreq->max_len		= ULONG_MAX;
-	subreq->max_nr_segs	= INT_MAX;
 	subreq->stream_nr	= stream->stream_nr;
+	subreq->io_iter		= *wreq_iter;
 
-	kenter("R=%x[%x]", wreq->debug_id, subreq->debug_index);
-
-	trace_netfs_sreq_ref(wreq->debug_id, subreq->debug_index,
-			     refcount_read(&subreq->ref),
-			     netfs_sreq_trace_new);
+	_enter("R=%x[%x]", wreq->debug_id, subreq->debug_index);
 
 	trace_netfs_sreq(subreq, netfs_sreq_trace_prepare);
 
+	stream->sreq_max_len	= UINT_MAX;
+	stream->sreq_max_segs	= INT_MAX;
 	switch (stream->source) {
 	case NETFS_UPLOAD_TO_SERVER:
 		netfs_stat(&netfs_n_wh_upload);
-		subreq->max_len = wreq->wsize;
+		stream->sreq_max_len = wreq->wsize;
 		break;
 	case NETFS_WRITE_TO_CACHE:
 		netfs_stat(&netfs_n_wh_write);
@@ -215,46 +229,42 @@ static void netfs_do_issue_write(struct netfs_io_stream *stream,
 {
 	struct netfs_io_request *wreq = subreq->rreq;
 
-	kenter("R=%x[%x],%zx", wreq->debug_id, subreq->debug_index, subreq->len);
+	_enter("R=%x[%x],%zx", wreq->debug_id, subreq->debug_index, subreq->len);
 
 	if (test_bit(NETFS_SREQ_FAILED, &subreq->flags))
-		return netfs_write_subrequest_terminated(subreq, subreq->error, false);
-
-	// TODO: Use encrypted buffer
-	if (test_bit(NETFS_RREQ_USE_IO_ITER, &wreq->flags)) {
-		subreq->io_iter = wreq->io_iter;
-		iov_iter_advance(&subreq->io_iter,
-				 subreq->start + subreq->transferred - wreq->start);
-		iov_iter_truncate(&subreq->io_iter,
-				 subreq->len - subreq->transferred);
-	} else {
-		iov_iter_xarray(&subreq->io_iter, ITER_SOURCE, &wreq->mapping->i_pages,
-				subreq->start + subreq->transferred,
-				subreq->len   - subreq->transferred);
-	}
+		return netfs_write_subrequest_terminated(subreq, subreq->error);
 
 	trace_netfs_sreq(subreq, netfs_sreq_trace_submit);
 	stream->issue_write(subreq);
 }
 
 void netfs_reissue_write(struct netfs_io_stream *stream,
-			 struct netfs_io_subrequest *subreq)
+			 struct netfs_io_subrequest *subreq,
+			 struct iov_iter *source)
 {
+	size_t size = subreq->len - subreq->transferred;
+
+	// TODO: Use encrypted buffer
+	subreq->io_iter = *source;
+	iov_iter_advance(source, size);
+	iov_iter_truncate(&subreq->io_iter, size);
+
+	subreq->retry_count++;
+	__clear_bit(NETFS_SREQ_MADE_PROGRESS, &subreq->flags);
 	__set_bit(NETFS_SREQ_IN_PROGRESS, &subreq->flags);
+	netfs_stat(&netfs_n_wh_retry_write_subreq);
 	netfs_do_issue_write(stream, subreq);
 }
 
-static void netfs_issue_write(struct netfs_io_request *wreq,
-			      struct netfs_io_stream *stream)
+void netfs_issue_write(struct netfs_io_request *wreq,
+		       struct netfs_io_stream *stream)
 {
 	struct netfs_io_subrequest *subreq = stream->construct;
 
 	if (!subreq)
 		return;
 	stream->construct = NULL;
-
-	if (subreq->start + subreq->len > wreq->start + wreq->submitted)
-		WRITE_ONCE(wreq->submitted, subreq->start + subreq->len - wreq->start);
+	subreq->io_iter.count = subreq->len;
 	netfs_do_issue_write(stream, subreq);
 }
 
@@ -264,19 +274,19 @@ static void netfs_issue_write(struct netfs_io_request *wreq,
  * we can avoid overrunning the credits obtained (cifs) and try to parallelise
  * content-crypto preparation with network writes.
  */
-int netfs_advance_write(struct netfs_io_request *wreq,
-			struct netfs_io_stream *stream,
-			loff_t start, size_t len, bool to_eof)
+size_t netfs_advance_write(struct netfs_io_request *wreq,
+			   struct netfs_io_stream *stream,
+			   loff_t start, size_t len, bool to_eof)
 {
 	struct netfs_io_subrequest *subreq = stream->construct;
 	size_t part;
 
 	if (!stream->avail) {
-		kleave("no write");
+		_leave("no write");
 		return len;
 	}
 
-	kenter("R=%x[%x]", wreq->debug_id, subreq ? subreq->debug_index : 0);
+	_enter("R=%x[%x]", wreq->debug_id, subreq ? subreq->debug_index : 0);
 
 	if (subreq && start != subreq->start + subreq->len) {
 		netfs_issue_write(wreq, stream);
@@ -287,13 +297,14 @@ int netfs_advance_write(struct netfs_io_request *wreq,
 		netfs_prepare_write(wreq, stream, start);
 	subreq = stream->construct;
 
-	part = min(subreq->max_len - subreq->len, len);
-	kdebug("part %zx/%zx %zx/%zx", subreq->len, subreq->max_len, part, len);
+	part = umin(stream->sreq_max_len - subreq->len, len);
+	_debug("part %zx/%zx %zx/%zx", subreq->len, stream->sreq_max_len, part, len);
 	subreq->len += part;
 	subreq->nr_segs++;
+	stream->submit_extendable_to -= part;
 
-	if (subreq->len >= subreq->max_len ||
-	    subreq->nr_segs >= subreq->max_nr_segs ||
+	if (subreq->len >= stream->sreq_max_len ||
+	    subreq->nr_segs >= stream->sreq_max_segs ||
 	    to_eof) {
 		netfs_issue_write(wreq, stream);
 		subreq = NULL;
@@ -314,12 +325,16 @@ static int netfs_write_folio(struct netfs_io_request *wreq,
 	struct netfs_io_stream *stream;
 	struct netfs_group *fgroup; /* TODO: Use this with ceph */
 	struct netfs_folio *finfo;
+	size_t iter_off = 0;
 	size_t fsize = folio_size(folio), flen = fsize, foff = 0;
 	loff_t fpos = folio_pos(folio), i_size;
 	bool to_eof = false, streamw = false;
 	bool debug = false;
 
-	kenter("");
+	_enter("");
+
+	if (rolling_buffer_make_space(&wreq->buffer) < 0)
+		return -ENOMEM;
 
 	/* netfs_perform_write() may shift i_size around the page or from out
 	 * of the page to beyond it, but cannot move i_size into or through the
@@ -329,7 +344,7 @@ static int netfs_write_folio(struct netfs_io_request *wreq,
 
 	if (fpos >= i_size) {
 		/* mmap beyond eof. */
-		kdebug("beyond eof");
+		_debug("beyond eof");
 		folio_start_writeback(folio);
 		folio_unlock(folio);
 		wreq->nr_group_rel += netfs_folio_written_back(folio);
@@ -363,7 +378,7 @@ static int netfs_write_folio(struct netfs_io_request *wreq,
 	}
 	flen -= foff;
 
-	kdebug("folio %zx %zx %zx", foff, flen, fsize);
+	_debug("folio %zx %zx %zx", foff, flen, fsize);
 
 	/* Deal with discontinuities in the stream of dirty pages.  These can
 	 * arise from a number of sources:
@@ -407,19 +422,26 @@ static int netfs_write_folio(struct netfs_io_request *wreq,
 	folio_unlock(folio);
 
 	if (fgroup == NETFS_FOLIO_COPY_TO_CACHE) {
-		if (!fscache_resources_valid(&wreq->cache_resources)) {
+		if (!cache->avail) {
 			trace_netfs_folio(folio, netfs_folio_trace_cancel_copy);
 			netfs_issue_write(wreq, upload);
 			netfs_folio_written_back(folio);
 			return 0;
 		}
 		trace_netfs_folio(folio, netfs_folio_trace_store_copy);
+	} else if (!upload->avail && !cache->avail) {
+		trace_netfs_folio(folio, netfs_folio_trace_cancel_store);
+		netfs_folio_written_back(folio);
+		return 0;
 	} else if (!upload->construct) {
 		trace_netfs_folio(folio, netfs_folio_trace_store);
 	} else {
 		trace_netfs_folio(folio, netfs_folio_trace_store_plus);
 	}
 
+	/* Attach the folio to the rolling buffer. */
+	rolling_buffer_append(&wreq->buffer, folio, 0);
+
 	/* Move the submission point forward to allow for write-streaming data
 	 * not starting at the front of the page.  We don't do write-streaming
 	 * with the cache as the cache requires DIO alignment.
@@ -429,15 +451,14 @@ static int netfs_write_folio(struct netfs_io_request *wreq,
 	 */
 	for (int s = 0; s < NR_IO_STREAMS; s++) {
 		stream = &wreq->io_streams[s];
-		stream->submit_max_len = fsize;
 		stream->submit_off = foff;
 		stream->submit_len = flen;
-		if ((stream->source == NETFS_WRITE_TO_CACHE && streamw) ||
+		if (!stream->avail ||
+		    (stream->source == NETFS_WRITE_TO_CACHE && streamw) ||
 		    (stream->source == NETFS_UPLOAD_TO_SERVER &&
 		     fgroup == NETFS_FOLIO_COPY_TO_CACHE)) {
 			stream->submit_off = UINT_MAX;
 			stream->submit_len = 0;
-			stream->submit_max_len = 0;
 		}
 	}
 
@@ -465,11 +486,17 @@ static int netfs_write_folio(struct netfs_io_request *wreq,
 			break;
 		stream = &wreq->io_streams[choose_s];
 
+		/* Advance the iterator(s). */
+		if (stream->submit_off > iter_off) {
+			rolling_buffer_advance(&wreq->buffer, stream->submit_off - iter_off);
+			iter_off = stream->submit_off;
+		}
+
+		atomic64_set(&wreq->issued_to, fpos + stream->submit_off);
+		stream->submit_extendable_to = fsize - stream->submit_off;
 		part = netfs_advance_write(wreq, stream, fpos + stream->submit_off,
 					   stream->submit_len, to_eof);
-		atomic64_set(&wreq->issued_to, fpos + stream->submit_off);
 		stream->submit_off += part;
-		stream->submit_max_len -= part;
 		if (part > stream->submit_len)
 			stream->submit_len = 0;
 		else
@@ -478,6 +505,8 @@ static int netfs_write_folio(struct netfs_io_request *wreq,
 			debug = true;
 	}
 
+	if (fsize > iter_off)
+		rolling_buffer_advance(&wreq->buffer, fsize - iter_off);
 	atomic64_set(&wreq->issued_to, fpos + fsize);
 
 	if (!debug)
@@ -487,11 +516,35 @@ static int netfs_write_folio(struct netfs_io_request *wreq,
 		for (int s = 0; s < NR_IO_STREAMS; s++)
 			netfs_issue_write(wreq, &wreq->io_streams[s]);
 
-	kleave(" = 0");
+	_leave(" = 0");
 	return 0;
 }
 
 /*
+ * End the issuing of writes, letting the collector know we're done.
+ */
+static void netfs_end_issue_write(struct netfs_io_request *wreq)
+{
+	bool needs_poke = true;
+
+	smp_wmb(); /* Write subreq lists before ALL_QUEUED. */
+	set_bit(NETFS_RREQ_ALL_QUEUED, &wreq->flags);
+
+	for (int s = 0; s < NR_IO_STREAMS; s++) {
+		struct netfs_io_stream *stream = &wreq->io_streams[s];
+
+		if (!stream->active)
+			continue;
+		if (!list_empty(&stream->subrequests))
+			needs_poke = false;
+		netfs_issue_write(wreq, stream);
+	}
+
+	if (needs_poke)
+		netfs_wake_collector(wreq);
+}
+
+/*
  * Write some of the pending data back to the server
  */
 int netfs_writepages(struct address_space *mapping,
@@ -502,10 +555,14 @@ int netfs_writepages(struct address_space *mapping,
 	struct folio *folio;
 	int error = 0;
 
-	if (wbc->sync_mode == WB_SYNC_ALL)
+	if (!mutex_trylock(&ictx->wb_lock)) {
+		if (wbc->sync_mode == WB_SYNC_NONE) {
+			netfs_stat(&netfs_n_wb_lock_skip);
+			return 0;
+		}
+		netfs_stat(&netfs_n_wb_lock_wait);
 		mutex_lock(&ictx->wb_lock);
-	else if (!mutex_trylock(&ictx->wb_lock))
-		return 0;
+	}
 
 	/* Need the first folio to be able to set up the op. */
 	folio = writeback_iter(mapping, wbc, NULL, &error);
@@ -518,14 +575,15 @@ int netfs_writepages(struct address_space *mapping,
 		goto couldnt_start;
 	}
 
+	__set_bit(NETFS_RREQ_OFFLOAD_COLLECTION, &wreq->flags);
 	trace_netfs_write(wreq, netfs_write_trace_writeback);
 	netfs_stat(&netfs_n_wh_writepages);
 
 	do {
-		kdebug("wbiter %lx %llx", folio->index, wreq->start + wreq->submitted);
+		_debug("wbiter %lx %llx", folio->index, atomic64_read(&wreq->issued_to));
 
 		/* It appears we don't have to handle cyclic writeback wrapping. */
-		WARN_ON_ONCE(wreq && folio_pos(folio) < wreq->start + wreq->submitted);
+		WARN_ON_ONCE(wreq && folio_pos(folio) < atomic64_read(&wreq->issued_to));
 
 		if (netfs_folio_group(folio) != NETFS_FOLIO_COPY_TO_CACHE &&
 		    unlikely(!test_bit(NETFS_RREQ_UPLOAD_TO_SERVER, &wreq->flags))) {
@@ -538,22 +596,20 @@ int netfs_writepages(struct address_space *mapping,
 			break;
 	} while ((folio = writeback_iter(mapping, wbc, folio, &error)));
 
-	for (int s = 0; s < NR_IO_STREAMS; s++)
-		netfs_issue_write(wreq, &wreq->io_streams[s]);
-	smp_wmb(); /* Write lists before ALL_QUEUED. */
-	set_bit(NETFS_RREQ_ALL_QUEUED, &wreq->flags);
+	netfs_end_issue_write(wreq);
 
 	mutex_unlock(&ictx->wb_lock);
+	netfs_wake_collector(wreq);
 
-	netfs_put_request(wreq, false, netfs_rreq_trace_put_return);
-	kleave(" = %d", error);
+	netfs_put_request(wreq, netfs_rreq_trace_put_return);
+	_leave(" = %d", error);
 	return error;
 
 couldnt_start:
 	netfs_kill_dirty_pages(mapping, wbc, folio);
 out:
 	mutex_unlock(&ictx->wb_lock);
-	kleave(" = %d", error);
+	_leave(" = %d", error);
 	return error;
 }
 EXPORT_SYMBOL(netfs_writepages);
@@ -590,8 +646,8 @@ int netfs_advance_writethrough(struct netfs_io_request *wreq, struct writeback_c
 			       struct folio *folio, size_t copied, bool to_page_end,
 			       struct folio **writethrough_cache)
 {
-	kenter("R=%x ic=%zu ws=%u cp=%zu tp=%u",
-	       wreq->debug_id, wreq->iter.count, wreq->wsize, copied, to_page_end);
+	_enter("R=%x ic=%zu ws=%u cp=%zu tp=%u",
+	       wreq->debug_id, wreq->buffer.iter.count, wreq->wsize, copied, to_page_end);
 
 	if (!*writethrough_cache) {
 		if (folio_test_dirty(folio))
@@ -618,31 +674,26 @@ int netfs_advance_writethrough(struct netfs_io_request *wreq, struct writeback_c
 /*
  * End a write operation used when writing through the pagecache.
  */
-int netfs_end_writethrough(struct netfs_io_request *wreq, struct writeback_control *wbc,
-			   struct folio *writethrough_cache)
+ssize_t netfs_end_writethrough(struct netfs_io_request *wreq, struct writeback_control *wbc,
+			       struct folio *writethrough_cache)
 {
 	struct netfs_inode *ictx = netfs_inode(wreq->inode);
-	int ret;
+	ssize_t ret;
 
-	kenter("R=%x", wreq->debug_id);
+	_enter("R=%x", wreq->debug_id);
 
 	if (writethrough_cache)
 		netfs_write_folio(wreq, wbc, writethrough_cache);
 
-	netfs_issue_write(wreq, &wreq->io_streams[0]);
-	netfs_issue_write(wreq, &wreq->io_streams[1]);
-	smp_wmb(); /* Write lists before ALL_QUEUED. */
-	set_bit(NETFS_RREQ_ALL_QUEUED, &wreq->flags);
+	netfs_end_issue_write(wreq);
 
 	mutex_unlock(&ictx->wb_lock);
 
-	if (wreq->iocb) {
+	if (wreq->iocb)
 		ret = -EIOCBQUEUED;
-	} else {
-		wait_on_bit(&wreq->flags, NETFS_RREQ_IN_PROGRESS, TASK_UNINTERRUPTIBLE);
-		ret = wreq->error;
-	}
-	netfs_put_request(wreq, false, netfs_rreq_trace_put_return);
+	else
+		ret = netfs_wait_for_write(wreq);
+	netfs_put_request(wreq, netfs_rreq_trace_put_return);
 	return ret;
 }
 
@@ -657,7 +708,7 @@ int netfs_unbuffered_write(struct netfs_io_request *wreq, bool may_wait, size_t
 	loff_t start = wreq->start;
 	int error = 0;
 
-	kenter("%zx", len);
+	_enter("%zx", len);
 
 	if (wreq->origin == NETFS_DIO_WRITE)
 		inode_dio_begin(wreq->inode);
@@ -665,25 +716,211 @@ int netfs_unbuffered_write(struct netfs_io_request *wreq, bool may_wait, size_t
 	while (len) {
 		// TODO: Prepare content encryption
 
-		kdebug("unbuffered %zx", len);
+		_debug("unbuffered %zx", len);
 		part = netfs_advance_write(wreq, upload, start, len, false);
 		start += part;
 		len -= part;
-		if (test_bit(NETFS_RREQ_PAUSE, &wreq->flags)) {
-			trace_netfs_rreq(wreq, netfs_rreq_trace_wait_pause);
-			wait_on_bit(&wreq->flags, NETFS_RREQ_PAUSE, TASK_UNINTERRUPTIBLE);
-		}
+		rolling_buffer_advance(&wreq->buffer, part);
+		if (test_bit(NETFS_RREQ_PAUSE, &wreq->flags))
+			netfs_wait_for_paused_write(wreq);
 		if (test_bit(NETFS_RREQ_FAILED, &wreq->flags))
 			break;
 	}
 
-	netfs_issue_write(wreq, upload);
+	netfs_end_issue_write(wreq);
+	_leave(" = %d", error);
+	return error;
+}
+
+/*
+ * Write some of a pending folio data back to the server and/or the cache.
+ */
+static int netfs_write_folio_single(struct netfs_io_request *wreq,
+				    struct folio *folio)
+{
+	struct netfs_io_stream *upload = &wreq->io_streams[0];
+	struct netfs_io_stream *cache  = &wreq->io_streams[1];
+	struct netfs_io_stream *stream;
+	size_t iter_off = 0;
+	size_t fsize = folio_size(folio), flen;
+	loff_t fpos = folio_pos(folio);
+	bool to_eof = false;
+	bool no_debug = false;
+
+	_enter("");
+
+	flen = folio_size(folio);
+	if (flen > wreq->i_size - fpos) {
+		flen = wreq->i_size - fpos;
+		folio_zero_segment(folio, flen, fsize);
+		to_eof = true;
+	} else if (flen == wreq->i_size - fpos) {
+		to_eof = true;
+	}
+
+	_debug("folio %zx/%zx", flen, fsize);
+
+	if (!upload->avail && !cache->avail) {
+		trace_netfs_folio(folio, netfs_folio_trace_cancel_store);
+		return 0;
+	}
+
+	if (!upload->construct)
+		trace_netfs_folio(folio, netfs_folio_trace_store);
+	else
+		trace_netfs_folio(folio, netfs_folio_trace_store_plus);
+
+	/* Attach the folio to the rolling buffer. */
+	folio_get(folio);
+	rolling_buffer_append(&wreq->buffer, folio, NETFS_ROLLBUF_PUT_MARK);
+
+	/* Move the submission point forward to allow for write-streaming data
+	 * not starting at the front of the page.  We don't do write-streaming
+	 * with the cache as the cache requires DIO alignment.
+	 *
+	 * Also skip uploading for data that's been read and just needs copying
+	 * to the cache.
+	 */
+	for (int s = 0; s < NR_IO_STREAMS; s++) {
+		stream = &wreq->io_streams[s];
+		stream->submit_off = 0;
+		stream->submit_len = flen;
+		if (!stream->avail) {
+			stream->submit_off = UINT_MAX;
+			stream->submit_len = 0;
+		}
+	}
 
+	/* Attach the folio to one or more subrequests.  For a big folio, we
+	 * could end up with thousands of subrequests if the wsize is small -
+	 * but we might need to wait during the creation of subrequests for
+	 * network resources (eg. SMB credits).
+	 */
+	for (;;) {
+		ssize_t part;
+		size_t lowest_off = ULONG_MAX;
+		int choose_s = -1;
+
+		/* Always add to the lowest-submitted stream first. */
+		for (int s = 0; s < NR_IO_STREAMS; s++) {
+			stream = &wreq->io_streams[s];
+			if (stream->submit_len > 0 &&
+			    stream->submit_off < lowest_off) {
+				lowest_off = stream->submit_off;
+				choose_s = s;
+			}
+		}
+
+		if (choose_s < 0)
+			break;
+		stream = &wreq->io_streams[choose_s];
+
+		/* Advance the iterator(s). */
+		if (stream->submit_off > iter_off) {
+			rolling_buffer_advance(&wreq->buffer, stream->submit_off - iter_off);
+			iter_off = stream->submit_off;
+		}
+
+		atomic64_set(&wreq->issued_to, fpos + stream->submit_off);
+		stream->submit_extendable_to = fsize - stream->submit_off;
+		part = netfs_advance_write(wreq, stream, fpos + stream->submit_off,
+					   stream->submit_len, to_eof);
+		stream->submit_off += part;
+		if (part > stream->submit_len)
+			stream->submit_len = 0;
+		else
+			stream->submit_len -= part;
+		if (part > 0)
+			no_debug = true;
+	}
+
+	wreq->buffer.iter.iov_offset = 0;
+	if (fsize > iter_off)
+		rolling_buffer_advance(&wreq->buffer, fsize - iter_off);
+	atomic64_set(&wreq->issued_to, fpos + fsize);
+
+	if (!no_debug)
+		kdebug("R=%x: No submit", wreq->debug_id);
+	_leave(" = 0");
+	return 0;
+}
+
+/**
+ * netfs_writeback_single - Write back a monolithic payload
+ * @mapping: The mapping to write from
+ * @wbc: Hints from the VM
+ * @iter: Data to write, must be ITER_FOLIOQ.
+ *
+ * Write a monolithic, non-pagecache object back to the server and/or
+ * the cache.
+ */
+int netfs_writeback_single(struct address_space *mapping,
+			   struct writeback_control *wbc,
+			   struct iov_iter *iter)
+{
+	struct netfs_io_request *wreq;
+	struct netfs_inode *ictx = netfs_inode(mapping->host);
+	struct folio_queue *fq;
+	size_t size = iov_iter_count(iter);
+	int ret;
+
+	if (WARN_ON_ONCE(!iov_iter_is_folioq(iter)))
+		return -EIO;
+
+	if (!mutex_trylock(&ictx->wb_lock)) {
+		if (wbc->sync_mode == WB_SYNC_NONE) {
+			netfs_stat(&netfs_n_wb_lock_skip);
+			return 0;
+		}
+		netfs_stat(&netfs_n_wb_lock_wait);
+		mutex_lock(&ictx->wb_lock);
+	}
+
+	wreq = netfs_create_write_req(mapping, NULL, 0, NETFS_WRITEBACK_SINGLE);
+	if (IS_ERR(wreq)) {
+		ret = PTR_ERR(wreq);
+		goto couldnt_start;
+	}
+
+	__set_bit(NETFS_RREQ_OFFLOAD_COLLECTION, &wreq->flags);
+	trace_netfs_write(wreq, netfs_write_trace_writeback_single);
+	netfs_stat(&netfs_n_wh_writepages);
+
+	if (__test_and_set_bit(NETFS_RREQ_UPLOAD_TO_SERVER, &wreq->flags))
+		wreq->netfs_ops->begin_writeback(wreq);
+
+	for (fq = (struct folio_queue *)iter->folioq; fq; fq = fq->next) {
+		for (int slot = 0; slot < folioq_count(fq); slot++) {
+			struct folio *folio = folioq_folio(fq, slot);
+			size_t part = umin(folioq_folio_size(fq, slot), size);
+
+			_debug("wbiter %lx %llx", folio->index, atomic64_read(&wreq->issued_to));
+
+			ret = netfs_write_folio_single(wreq, folio);
+			if (ret < 0)
+				goto stop;
+			size -= part;
+			if (size <= 0)
+				goto stop;
+		}
+	}
+
+stop:
+	for (int s = 0; s < NR_IO_STREAMS; s++)
+		netfs_issue_write(wreq, &wreq->io_streams[s]);
 	smp_wmb(); /* Write lists before ALL_QUEUED. */
 	set_bit(NETFS_RREQ_ALL_QUEUED, &wreq->flags);
-	if (list_empty(&upload->subrequests))
-		netfs_wake_write_collector(wreq, false);
 
-	kleave(" = %d", error);
-	return error;
+	mutex_unlock(&ictx->wb_lock);
+	netfs_wake_collector(wreq);
+
+	netfs_put_request(wreq, netfs_rreq_trace_put_return);
+	_leave(" = %d", ret);
+	return ret;
+
+couldnt_start:
+	mutex_unlock(&ictx->wb_lock);
+	_leave(" = %d", ret);
+	return ret;
 }
+EXPORT_SYMBOL(netfs_writeback_single);