On Fri, Apr 27, 2018 at 03:05:24PM +0800, Ethan Lien wrote: > We should balance dirty metadata pages at the end of > btrfs_finish_ordered_io, since a small, unmergeable random write can > potentially produce dirty metadata which is multiple times larger than > the data itself. For example, a small, unmergeable 4KiB write may > produce: > > 16KiB dirty leaf (and possibly 16KiB dirty node) in subvolume tree > 16KiB dirty leaf (and possibly 16KiB dirty node) in checksum tree > 16KiB dirty leaf (and possibly 16KiB dirty node) in extent tree > > Although we do call balance dirty pages in write side, but in the > buffered write path, most metadata are dirtied only after we reach the > dirty background limit (which by far onlys counts dirty data pages) and > wakeup the flusher thread. If there are many small, unmergeable random > writes spread in a large btree, we'll find a burst of dirty pages > exceeds the dirty_bytes limit after we wakeup the flusher thread - which > is not what we expect. In our machine, it caused out-of-memory problem > since a page cannot be dropped if it is marked dirty. > > Someone may worry about we may sleep in btrfs_btree_balance_dirty(), but > since we do btrfs_finish_ordered_io in a separate worker, it will not > stop the flusher consuming dirty pages. Also, we use different worker for > metadata writeback endio, sleep in btrfs_finish_ordered_io help us > throttle the size of dirty metadata pages. The described scenario sounds interesting. Do you have some scripted steps to reproduce it? btrfs_btree_balance_dirty detects congestion and can skip the balancing eventually, so the case when it actually does something and waits is the point where things can go bad. From the last paragraph, it is clear that you have considered that, that's good. Have you also considered calling __btrfs_btree_balance_dirty with flush_delayed=0 ? This would avoid the waiting and I'm not sure if it's really required here to get out of the situation. Anyway, I'll add the patch to for-next for more testing. -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
