Re: [PATCH 3/5] writeback: prevent sync livelock with the sync_after timestamp

Wu Fengguang
2010-07-30T05:17:46Z

On Thu, Jul 29, 2010 at 11:02:41PM +0800, Jan Kara wrote:
> Hi Fengguang,
>
> On Thu 29-07-10 19:51:45, Wu Fengguang wrote:
> > The start time in writeback_inodes_wb() is not very useful because it
> > slips at each invocation time. Preferrably one _constant_ time shall be
> > used at the beginning to cover the whole sync() work.
> >
> > The newly dirtied inodes are now guarded at the queue_io() time instead
> > of the b_io walk time. This is more natural: non-empty b_io/b_more_io
> > means "more work pending".
> >
> > The timestamp is now grabbed the sync work submission time, and may be
> > further optimized to the initial sync() call time.
> The patch seems to have some issues...
>
> > + if (wbc->for_sync) {
> For example this is never set. You only set wb->for_sync.

Ah right.

> > + expire_interval = 1;
> > + older_than_this = wbc->sync_after;
> And sync_after is never set either???

Sorry I must lose some chunk when rebasing the patch ..

> > - if (!(wbc->for_kupdate || wbc->for_background) || list_empty(&wb->b_io))
> > + if (list_empty(&wb->b_io))
> > queue_io(wb, wbc);
> And what is the purpose of this? It looks as an unrelated change to me.

Yes it's not tightly related. It may be simpler to do

- if (!wbc->for_kupdate || list_empty(&wb->b_io))
+ if (list_empty(&wb->b_io))

in the previous patch "writeback: sync expired inodes first in
background writeback".

Thanks,
Fengguang