documentation: document the is_dirty_writeback aops callback

Signed-off-by: NMel Gorman <mgorman@suse.de>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

Documentation/filesystems/vfs.txt

@@ -582,6 +582,7 @@ struct address_space_operations {
 	int (*launder_page) (struct page *);
 	int (*is_partially_uptodate) (struct page *, read_descriptor_t *,
 					unsigned long);
+	void (*is_dirty_writeback) (struct page *, bool *, bool *);
 	int (*error_remove_page) (struct mapping *mapping, struct page *page);
 	int (*swap_activate)(struct file *);
 	int (*swap_deactivate)(struct file *);
@@ -746,6 +747,15 @@ struct address_space_operations {
 	block is up to date then the read can complete without needing the IO
 	to bring the whole page up to date.
 
+  is_dirty_writeback: Called by the VM when attempting to reclaim a page.
+	The VM uses dirty and writeback information to determine if it needs
+	to stall to allow flushers a chance to complete some IO. Ordinarily
+	it can use PageDirty and PageWriteback but some filesystems have
+	more complex state (unstable pages in NFS prevent reclaim) or
+	do not set those flags due to locking problems (jbd). This callback
+	allows a filesystem to indicate to the VM if a page should be
+	treated as dirty or writeback for the purposes of stalling.
+
   error_remove_page: normally set to generic_error_remove_page if truncation
 	is ok for this address space. Used for memory failure handling.
 	Setting this implies you deal with pages going away under you,