centos man page for journal_get_undo_access

JOURNAL_GET_UNDO_ACC(9) 				     The Linux Journalling API					   JOURNAL_GET_UNDO_ACC(9)

NAME
       journal_get_undo_access - Notify intent to modify metadata with non-rewindable consequences

SYNOPSIS
       int journal_get_undo_access(handle_t * handle, struct buffer_head * bh);

ARGUMENTS
       handle
	   transaction

       bh
	   buffer to undo

DESCRIPTION
       Sometimes there is a need to distinguish between metadata which has been committed to disk and that which has not. The ext3fs code uses
       this for freeing and allocating space, we have to make sure that we do not reuse freed space until the deallocation has been committed,
       since if we overwrote that space we would make the delete un-rewindable in case of a crash.

       To deal with that, journal_get_undo_access requests write access to a buffer for parts of non-rewindable operations such as delete
       operations on the bitmaps. The journaling code must keep a copy of the buffer's contents prior to the undo_access call until such time as
       we know that the buffer has definitely been committed to disk.

       We never need to know which transaction the committed data is part of, buffers touched here are guaranteed to be dirtied later and so will
       be committed to a new transaction in due course, at which point we can discard the old committed data pointer.

       Returns error number or 0 on success.

AUTHORS
       Roger Gammans <rgammans@computer-surgery.co.uk>
	   Author.

       Stephen Tweedie <sct@redhat.com>
	   Author.

COPYRIGHT
Kernel Hackers Manual 3.10					     June 2014						   JOURNAL_GET_UNDO_ACC(9)