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jbd2_journal_get_undo_access(9) - phpMan

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JBD2_JOURNAL_GET_UND(9)             The Linux Journalling API             JBD2_JOURNAL_GET_UND(9)



NAME
       jbd2_journal_get_undo_access - Notify intent to modify metadata with non-rewindable
       consequences

SYNOPSIS
       int jbd2_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, jbd2_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 AT computer-surgery.uk>
           Author.

       Stephen Tweedie <sct AT redhat.com>
           Author.

COPYRIGHT
Kernel Hackers Manual 4.8.                 January 2017                   JBD2_JOURNAL_GET_UND(9)


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