Re: [PATCH 1/2] writeback: Improve busyloop prevention

[Christoph Hellwig <hch@infradead.org>]

On Tue, Oct 18, 2011 at 02:51:28AM +0200, Jan Kara wrote:
> > It's still puzzling why dirty pages remain at 286 and does not get
> > cleaned by either flusher threads for local XFS and NFSROOT for so
> > long time..
>   I was looking at this as well. So the reason why pages were not cleaned
> by the flusher thread is that there were 2 dirty inodes and the inode with
> dirty pages had i_dirtied_whan newer than the time when we started this
> background writeback. Thus the running background writeback work always
> included only the other inode which has no dirty pages but I_DIRTY_SYNC set.
> Apparently XFS is stubborn and refuses to write the inode although we try
> rather hard. That is probably because dd writing to this inode is stuck in
> balance_dirty_pages() and holds ilock - which is a bit unfortunate behavior
> but what can we do...

Stop writing data from balance_dirty_pages()?

Anyway, XFS tries very hard to not block in a non-block ->write_inode
or ->writepages, which generally is a good thing to avoid getting stuck
in the flusher thread.  For cases like this where an inode is long
beyond it's due time it might make sense to simply do a synchronous
write_inode from the flusher thread to force the inode out.