Re: [PATCH 01/17] writeback: introduce .tagged_sync for the WB_SYNC_NONE sync stage

[Dave Chinner <david@fromorbit.com>]

On Fri, May 13, 2011 at 10:56:08AM +0800, Wu Fengguang wrote:
> On Fri, May 13, 2011 at 06:40:13AM +0800, Dave Chinner wrote:
> > On Thu, May 12, 2011 at 09:57:07PM +0800, 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 also need livelock prevention.
> > > 
> > > Impact:  It changes the order in which pages/inodes are synced to disk.
> > > Now in the WB_SYNC_NONE stage, it won't proceed to write the next inode
> > > until finished with the current inode.
> > 
> > What about all the filesystems that implement their own
> > .writepages()/write_cache_pages() functions or have
> > have special code that checks WB_SYNC_ALL in .writepages (e.g. gfs2,
> > ext4, btrfs and perhaps others). Don't they all need to be aware of
> > this tagged_sync field?
> 
> Right, good point. Currently only ext4 is updated. The other
> filesystems --- afs, btrfs, cifs, gfs2 --- do not even use
> PAGECACHE_TAG_TOWRITE for livelock prevention. My plan was to add
> PAGECACHE_TAG_TOWRITE and tagged_sync code to them as the next step,
> when tagged_sync is accepted and proved to work fine.

Where "proved to work fine" can mean "caused regressions for certain
filesystems"? I mean, for btrfs it means that the bio is submitted
with WRITE rather than WRITE_SYNC, which causes subtle changes of
behaviour in the elevator. that could cause strange regressions that
are very hard to isolate.

Hence regardless of whether filesystems use PAGECACHE_TAG_TOWRITE
or not, filesystems are checking for synchronous writeback for
a reason. If we now have two different ways of signalling sync
writeback they need to know about them.

Which just raised the question in my mind - why did you add a new
field rather than a new sync_mode definition? After all, this is a
new sync control, and it seems clumsy to me to have two separate
control fields for defining sync behaviour...

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com