Re: [PATCH] mm,writeback: Don't use memory reserves for wb_start_writeback

[Jan Kara <jack@suse.cz>]

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@suse.com>
SUSE Labs, CR

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