Re: [PATCH 1/3] writeback: introduce wbc.tagged_sync for the WB_SYNC_NONE sync stage

[Jan Kara <jack@suse.cz>]

On Thu 05-05-11 20:14:02, Wu Fengguang wrote:
> On Thu, May 05, 2011 at 05:00:59AM +0800, Jan Kara wrote:
> > On Mon 02-05-11 11:17:51, Wu Fengguang wrote:
> > > sync(2) is performed in two stages: the WB_SYNC_NONE sync and the
> > > WB_SYNC_ALL sync. Tag the first stage with wbc.tagged_sync and do
> > > livelock prevention for it, too.
> > > 
> > > Note that writeback_inodes_sb() is called by not only sync(), they are
> > > treated the same because the other callers need also need livelock
> > > prevention.
> >   I was thinking about this and could not find any - which other callers
> > of writeback_inodes_sb() need the livelock prevention?
> 
> For example, the writeback_inodes_sb_if_idle() call from ext4.
> In general anyone that pass get_nr_dirty_pages() as work->nr_pages
> may be highly over-estimating the work set.
  OK, I see what you mean. I agree using tagging in these cases probably
makes sense.

> It won't be directly livelocked since ext4 won't wait for completion,
> however there is possibility the works queued behind are delayed and
> livelocked.
  Actually it will, writeback_inodes_sb() waits for completion (because of
s_umount locking). ext4 should use writeback_inodes_sb_if_idle_nr() (with
some relatively small number) anyway to avoid the pauses.

> Ideally simple ->nr_pages works should be given lower priority and
> even may be merged with each other, and that would be future work.
  Merging works wanting to do the same thing would be nice. I though about
it some time ago for a while but getting all the combinations right and
making the merging code resonably simple was hard so I postponed it for
later because it was not urgent.

								Honza
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR