+Writeback makes use of a writeback_control structure to direct the
+operations. This gives the the writepage and writepages operations some
+information about the nature of and reason for the writeback request,
+and the constraints under which it is being done. It is also used to
+return information back to the caller about the result of a writepage or
+writepages request.
+
+Handling errors during writeback
+--------------------------------
+Most applications that do buffered I/O will periodically call a file
+synchronization call (fsync, fdatasync, msync or sync_file_range) to
+ensure that data written has made it to the backing store. When there
+is an error during writeback, they expect that error to be reported when
+a file sync request is made. After an error has been reported on one
+request, subsequent requests on the same file descriptor should return
+0, unless further writeback errors have occurred since the previous file
+syncronization.
+
+Ideally, the kernel would report errors only on file descriptions on
+which writes were done that subsequently failed to be written back. The
+generic pagecache infrastructure does not track the file descriptions
+that have dirtied each individual page however, so determining which
+file descriptors should get back an error is not possible.
+
+Instead, the generic writeback error tracking infrastructure in the
+kernel settles for reporting errors to fsync on all file descriptions
+that were open at the time that the error occurred. In a situation with
+multiple writers, all of them will get back an error on a subsequent fsync,
+even if all of the writes done through that particular file descriptor
+succeeded (or even if there were no writes on that file descriptor at all).
+
+Filesystems that wish to use this infrastructure should call
+mapping_set_error to record the error in the address_space when it
+occurs. Then, after writing back data from the pagecache in their
+file->fsync operation, they should call file_check_and_advance_wb_err to
+ensure that the struct file's error cursor has advanced to the correct
+point in the stream of errors emitted by the backing device(s).