On Wed 16-03-16 13:46:17, Tejun Heo wrote:
> Hello,
>
> (cc'ing Jan)
>
> On Mon, Mar 14, 2016 at 05:09:00PM +0100, Michal Hocko wrote:
> > On Sun 13-03-16 23:22:23, Tetsuo Handa wrote:
> > [...]
> >
> > I am not familiar with the writeback code so I might be missing
> > something essential here but why are we even queueing more and more
> > work without checking there has been enough already scheduled or in
> > progress.
> >
> > Something as simple as:
> > diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> > index 6915c950e6e8..aa52e23ac280 100644
> > --- a/fs/fs-writeback.c
> > +++ b/fs/fs-writeback.c
> > @@ -887,7 +887,7 @@ void wb_start_writeback(struct bdi_writeback *wb, long nr_pages,
> > {
> > struct wb_writeback_work *work;
> >
> > - if (!wb_has_dirty_io(wb))
> > + if (!wb_has_dirty_io(wb) || writeback_in_progress(wb))
> > return;
>
> I'm not sure this would be safe. It shouldn't harm correctness as
> wb_start_writeback() isn't used in sync case but this might change
> flush behavior in various ways. Dropping GFP_ATOMIC as suggested by
> Tetsuo is likely better.
Yes, there can be different requests for different numbers of pages to be
written and you don't want to discard a request to clean 4000 pages just
because a writeback of 10 pages is just running. As Tejun says, this is not
a hard requirement but in general it would be unexpected for the users of
the api...
Honza
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
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