journal_get_undo_access(9) [centos man page]

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)

JOURNAL_TRY_TO_FREE_(9) 				     The Linux Journalling API					   JOURNAL_TRY_TO_FREE_(9)

NAME

       journal_try_to_free_buffers - try to free page buffers.

SYNOPSIS

       int journal_try_to_free_buffers(journal_t * journal, struct page * page, gfp_t gfp_mask);

ARGUMENTS

       journal
	   journal for operation

       page
	   to try and free

       gfp_mask
	   we use the mask to detect how hard should we try to release buffers. If __GFP_WAIT and __GFP_FS is set, we wait for commit code to
	   release the buffers.

DESCRIPTION

       For all the buffers on this page, if they are fully written out ordered data, move them onto BUF_CLEAN so try_to_free_buffers can reap
       them.

       This function returns non-zero if we wish try_to_free_buffers to be called. We do this if the page is releasable by try_to_free_buffers. We
       also do it if the page has locked or dirty buffers and the caller wants us to perform sync or async writeout.

       This complicates JBD locking somewhat. We aren't protected by the BKL here. We wish to remove the buffer from its committing or running
       transaction's ->t_datalist via __journal_unfile_buffer.

       This may *change* the value of transaction_t->t_datalist, so anyone who looks at t_datalist needs to lock against this function.

       Even worse, someone may be doing a journal_dirty_data on this buffer. So we need to lock against that.  journal_dirty_data will come out of
       the lock with the buffer dirty, which makes it ineligible for release here.

       Who else is affected by this? hmm... Really the only contender is do_get_write_access - it could be looking at the buffer while
       journal_try_to_free_buffer is changing its state. But that cannot happen because we never reallocate freed data as metadata while the data
       is part of a transaction. Yes?

       Return 0 on failure, 1 on success

AUTHORS

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

       Stephen Tweedie <sct@redhat.com>
	   Author.

COPYRIGHT

Kernel Hackers Manual 2.6.					     July 2010						   JOURNAL_TRY_TO_FREE_(9)