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.TH "JOURNAL_GET_UNDO_ACCESS" "9" "17 July 2003" "" ""

.SH NAME
journal_get_undo_access \- Notify intent to modify metadata with
.SH SYNOPSIS
.sp
\fB
.sp
int journal_get_undo_access  (handle_t * \fIhandle\fB, struct buffer_head * \fIbh\fB, int * \fIcredits\fB);
\fR
.SH "ARGUMENTS"
.TP
\fB\fIhandle\fB\fR
transaction
.TP
\fB\fIbh\fB\fR
buffer to undo
.TP
\fB\fIcredits\fB\fR
store the number of taken credits here (if not NULL)
.SH "DESCRIPTION"
.PP
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.
.PP
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.
.PP
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.
.PP
Returns error number or 0 on success.
.SH "DESCRIPTION"
.PP
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.
.PP
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.
.PP
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.
.PP
Returns error number or 0 on success.
.SH "ABOUT THIS DOCUMENT"
.PP
This documentation is free software; you can redistribute
it and/or modify it under the terms of the GNU General Public
License as published by the Free Software Foundation; either
version 2 of the License, or (at your option) any later
version.
.PP
This program is distributed in the hope that it will be
useful, but WITHOUT ANY WARRANTY; without even the implied
warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
See the GNU General Public License for more details.
.PP
You should have received a copy of the GNU General Public
License along with this program; if not, write to the Free
Software Foundation, Inc., 59 Temple Place, Suite 330, Boston,
MA 02111-1307 USA
.PP
For more details see the file COPYING in the source
distribution of Linux.
.PP
If you have comments on the formatting of this manpage, then please contact
Michael Still (mikal@stillhq.com).
.PP
This documentation was generated with kernel version 2.5.73.