int journal_try_to_free_buffers (journal_t * journal, struct page * page, int unused_gfp_mask);
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?