Documentation: flesh out the section in vfs.txt on storing and reporting writeback errors

written at any point after PG_Dirty is clear.  Once it is known to be

safe, PG_Writeback is cleared.

Writeback makes use of a writeback_control structure...

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).

struct address_space_operations

-------------------------------

The File Object

===============

A file object represents a file opened by a process.

A file object represents a file opened by a process. This is also known

as an "open file description" in POSIX parlance.

struct file_operations

  release: called when the last reference to an open file is closed

  fsync: called by the fsync(2) system call

  fsync: called by the fsync(2) system call. Also see the section above

	 entitled "Handling errors during writeback".

  fasync: called by the fcntl(2) system call when asynchronous

	(non-blocking) mode is enabled for a file