On Thu 01-03-12 20:36:40, Wu Fengguang wrote: > > Please have a think about all of this and see if you can demonstrate > > how the iput() here is guaranteed safe. > > There are already several __iget()/iput() calls inside fs-writeback.c. > The existing iput() calls already demonstrate its safety? > > Basically the flusher works in this way > > - the dirty inode list i_wb_list does not reference count the inode at all > > - the flusher thread does something analog to igrab() and set I_SYNC > before going off to writeout the inode > > - evict() will wait for completion of I_SYNC Yes, you are right that currently writeback code already holds inode references and so it can happen that flusher thread drops the last inode reference. But currently that could create problems only if someone waits for flusher thread to make progress while effectively blocking e.g. truncate from happening. Currently flusher thread handles sync(2) and background writeback and filesystems take care to not hold any locks blocking IO / truncate while possibly waiting for these. But with your addition situation changes significantly - now anyone doing allocation can block and do allocation from all sorts of places including ones where we hold locks blocking other fs activity. The good news is that we use GFP_NOFS in such places. So if GFP_NOFS allocation cannot possibly depend on a completion of some writeback work, then I'd still be comfortable with dropping inode references from writeback code. But Andrew is right this at least needs some arguing... Honza -- Jan Kara <jack@xxxxxxx> SUSE Labs, CR -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>