netfs: Slice a writeback from a dirty region

Handle the initiation of writeback of a piece of the dirty list.  A slice
of a dirty region is extracted and a writeback record is set up to manage
it.  The pages in the affected region are flipped from dirty to
writeback-in-progress.

The writeback is then dispatched (which currently just logs a "--- WRITE
---" message to dmesg and then abandons it).

Notes:

 (*) A page may host multiple disjoint dirty regions, each with its own
     netfs_dirty_region, and a region may span multiple pages.  Dirty
     regions are not permitted to overlap, though they may be merged if
     they would otherwise overlap.

 (*) A page may be involved in multiple simultaneous writebacks.  Each one
     is managed by a separate netfs_dirty_region and netfs_write_request.

 (*) Multiple pages may be required to form a write (for crypto/compression
     purposes) and so adjacent non-dirty pages may also get marked for
     writeback.

Signed-off-by: David Howells <dhowells@redhat.com>

fs/afs/file.c

@@ -55,7 +55,7 @@ const struct address_space_operations afs_file_aops = {
 	.releasepage	= netfs_releasepage,
 	.invalidatepage	= netfs_invalidatepage,
 	.writepage	= afs_writepage,
-	.writepages	= afs_writepages,
+	.writepages	= netfs_writepages,
 };
 
 const struct address_space_operations afs_symlink_aops = {
@@ -384,6 +384,11 @@ static void afs_free_dirty_region(struct netfs_dirty_region *region)
 	key_put(region->netfs_priv);
 }
 
+static void afs_init_writeback(struct netfs_writeback *wback)
+{
+	//wback->netfs_priv = key_get(afs_file_key(file));
+}
+
 static void afs_update_i_size(struct file *file, loff_t new_i_size)
 {
 	struct afs_vnode *vnode = AFS_FS_I(file_inode(file));
@@ -414,6 +419,7 @@ const struct netfs_request_ops afs_req_ops = {
 	.free_dirty_region	= afs_free_dirty_region,
 	.update_i_size		= afs_update_i_size,
 	.validate_for_write	= afs_validate_for_write,
+	.init_writeback		= afs_init_writeback,
 };
 
 int afs_write_inode(struct inode *inode, struct writeback_control *wbc)

fs/netfs/Makefile

@@ -7,6 +7,7 @@ netfs-y := \
 	misc.o \
 	objects.o \
 	read_helper.o \
+	write_back.o \
 	write_helper.o
 
 netfs-$(CONFIG_NETFS_STATS) += stats.o