Re: [PATCH 2/5] writeback: stop periodic/background work on seeing sync works

Wu Fengguang
2010-08-03T13:48:54Z

On Tue, Aug 03, 2010 at 09:44:31PM +0800, Wu Fengguang wrote:
> On Tue, Aug 03, 2010 at 09:22:16PM +0800, Wu Fengguang wrote:
> > > > Fengguang, how about merging also the attached simple patch together with
> > > > my fix? With these two patches, I'm not able to trigger any sync livelock
> > > > while without one of them I hit them quite easily...
> > >
> > > This looks OK. However note that redirty_tail() can modify
> > > dirtied_when unexpectedly. So the more we rely on wb_start, the more
> > > possibility an inode is (wrongly) skipped by sync. I have a bunch of
> > > patches to remove redirty_tail(). However they may not be good
> > > candidates for 2.6.36..
> >
> > It looks that setting wb_start at the beginning of
> > writeback_inodes_wb() won't be easily affected by redirty_tail().
>
> Except for this redirty_tail(), which may mess up the dirtied_when
> ordering in b_dirty and later on break the assumption of
> inode_dirtied_after(inode, wbc->wb_start).

Oh well, it seems still OK (it does sound crazy). Please ignore the
below patch, sorry for the noise..

Thanks,
Fengguang

> It can be replaced by a requeue_io() for now. Christoph mentioned a
> patchset to introduce sb->s_wb, which should be a better solution.
>
> Thanks,
> Fengguang
>
> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> index a178828..e56e68b 100644
> --- a/fs/fs-writeback.c
> +++ b/fs/fs-writeback.c
> @@ -457,12 +457,7 @@ int generic_writeback_sb_inodes(struct super_block *sb,
>
> if (inode->i_sb != sb) {
> if (only_this_sb) {
> - /*
> - * We only want to write back data for this
> - * superblock, move all inodes not belonging
> - * to it back onto the dirty list.
> - */
> - redirty_tail(inode);
> + requeue_io(inode);
> continue;
> }