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

[Wu Fengguang <fengguang.wu@intel.com>]

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.

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.

Ideally simple ->nr_pages works should be given lower priority and
even may be merged with each other, and that would be future work.

Thanks,
Fengguang