netfs: Provide an iterator-reset function

Provide a function to reset the iterator on a subrequest.
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Jeff Layton <jlayton@kernel.org>
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
Link: https://lore.kernel.org/r/20240814203850.2240469-17-dhowells@redhat.com/ # v2
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/netfs/internal.h

@@ -69,6 +69,7 @@ int netfs_buffer_append_folio(struct netfs_io_request *rreq, struct folio *folio
 			      bool needs_put);
 struct folio_queue *netfs_delete_buffer_head(struct netfs_io_request *wreq);
 void netfs_clear_buffer(struct netfs_io_request *rreq);
+void netfs_reset_iter(struct netfs_io_subrequest *subreq);
 
 /*
  * objects.c
@@ -161,9 +162,6 @@ struct netfs_io_request *netfs_create_write_req(struct address_space *mapping,
 void netfs_reissue_write(struct netfs_io_stream *stream,
 			 struct netfs_io_subrequest *subreq,
 			 struct iov_iter *source);
-int 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_request *netfs_begin_writethrough(struct kiocb *iocb, size_t len);
 int netfs_advance_writethrough(struct netfs_io_request *wreq, struct writeback_control *wbc,
 			       struct folio *folio, size_t copied, bool to_page_end,

fs/netfs/misc.c

@@ -84,6 +84,24 @@ void netfs_clear_buffer(struct netfs_io_request *rreq)
 	}
 }
 
+/*
+ * Reset the subrequest iterator to refer just to the region remaining to be
+ * read.  The iterator may or may not have been advanced by socket ops or
+ * extraction ops to an extent that may or may not match the amount actually
+ * read.
+ */
+void netfs_reset_iter(struct netfs_io_subrequest *subreq)
+{
+	struct iov_iter *io_iter = &subreq->io_iter;
+	size_t remain = subreq->len - subreq->transferred;
+
+	if (io_iter->count > remain)
+		iov_iter_advance(io_iter, io_iter->count - remain);
+	else if (io_iter->count < remain)
+		iov_iter_revert(io_iter, remain - io_iter->count);
+	iov_iter_truncate(&subreq->io_iter, remain);
+}
+
 /**
  * netfs_dirty_folio - Mark folio dirty and pin a cache object for writeback
  * @mapping: The mapping the folio belongs to.

fs/netfs/write_collect.c

@@ -226,9 +226,8 @@ static void netfs_retry_write_stream(struct netfs_io_request *wreq,
 		/* Determine the set of buffers we're going to use.  Each
 		 * subreq gets a subset of a single overall contiguous buffer.
 		 */
+		netfs_reset_iter(from);
 		source = from->io_iter;
-		iov_iter_revert(&source, subreq->len - source.count);
-		iov_iter_advance(&source, from->transferred);
 		source.count = len;
 
 		/* Work through the sublist. */

fs/netfs/write_issue.c

@@ -261,7 +261,7 @@ 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,
+static int netfs_advance_write(struct netfs_io_request *wreq,
 			       struct netfs_io_stream *stream,
 			       loff_t start, size_t len, bool to_eof)
 {