[RFC 17/19] netfs: support on demand read

[Jeffle Xu <jefflexu@linux.alibaba.com>]

Add demand_read() callback to netfs_cache_ops to implement demand read.

To implement demand-read semantics, the data blob file is a sparse file
on the first beginning. When fs starts to access the data blob file, it
will "cache miss" (hit the  hole) and then .issue_op() callback will be
called to prepare the data.

.issue_op() callback could call netfs_demand_read() helper function
introduced in this patch to prepare the data, which will in turn call
.demand_read() callback of fscache backend.

Signed-off-by: Jeffle Xu <jefflexu@linux.alibaba.com>
---
 fs/netfs/read_helper.c | 26 ++++++++++++++++++++++++++
 include/linux/netfs.h  |  4 ++++
 2 files changed, 30 insertions(+)

diff --git a/fs/netfs/read_helper.c b/fs/netfs/read_helper.c
index ade1523fc180..3460cfd7a570 100644
--- a/fs/netfs/read_helper.c
+++ b/fs/netfs/read_helper.c
@@ -1125,6 +1125,32 @@ int netfs_readpage_demand(struct folio *folio,
 }
 EXPORT_SYMBOL(netfs_readpage_demand);
 
+void netfs_demand_read(struct netfs_read_subrequest *subreq)
+{
+	struct netfs_read_request *rreq = subreq->rreq;
+	struct netfs_cache_resources *cres = &rreq->cache_resources;
+	loff_t start_pos;
+	size_t len;
+	int ret;
+
+	start_pos = subreq->start + subreq->transferred;
+	len = subreq->len - subreq->transferred;
+
+	/*
+	 * In success case (ret == 0), user daemon has downloaded data for us,
+	 * thus transform to NETFS_READ_FROM_CACHE state and advertise that 0
+	 * byte readed, so that the request will enter into INCOMPLETE state and
+	 * re-read from backing file.
+	 */
+	ret = cres->ops->demand_read(cres, start_pos, len);
+	if (!ret) {
+		subreq->source = NETFS_READ_FROM_CACHE;
+		__clear_bit(NETFS_SREQ_WRITE_TO_CACHE, &subreq->flags);
+	}
+
+	netfs_subreq_terminated(subreq, ret, false);
+}
+
 /*
  * Prepare a folio for writing without reading first
  * @folio: The folio being prepared
diff --git a/include/linux/netfs.h b/include/linux/netfs.h
index 5f45eb31defd..8a9dae361f07 100644
--- a/include/linux/netfs.h
+++ b/include/linux/netfs.h
@@ -253,6 +253,9 @@ struct netfs_cache_ops {
 	int (*prepare_write)(struct netfs_cache_resources *cres,
 			     loff_t *_start, size_t *_len, loff_t i_size,
 			     bool no_space_allocated_yet);
+
+	int (*demand_read)(struct netfs_cache_resources *cres,
+			   loff_t start_pos, size_t len);
 };
 
 struct readahead_control;
@@ -271,6 +274,7 @@ extern int netfs_write_begin(struct file *, struct address_space *,
 			     void **,
 			     const struct netfs_read_request_ops *,
 			     void *);
+extern void netfs_demand_read(struct netfs_read_subrequest *);
 
 extern void netfs_subreq_terminated(struct netfs_read_subrequest *, ssize_t, bool);
 extern void netfs_stats_show(struct seq_file *);
-- 
2.27.0