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journal_get_undo_access(9) [centos man page]

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

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)

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

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)
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