Re: [PATCH 8/8] writeback: Do not sleep on the congestion queue if there are no congested BDIs or if significant congestion is not being encountered in the current zone

[Mel Gorman <mel@csn.ul.ie>]

> > > <snip>
> > > 
> > > >  			      struct scan_control *sc)
> > > >  {
> > > > +	enum bdi_queue_status ret = QUEUEWRITE_DENIED;
> > > > +
> > > >  	if (current->flags & PF_SWAPWRITE)
> > > > -		return 1;
> > > > +		return QUEUEWRITE_ALLOWED;
> > > >  	if (!bdi_write_congested(bdi))
> > > > -		return 1;
> > > > +		return QUEUEWRITE_ALLOWED;
> > > > +	else
> > > > +		ret = QUEUEWRITE_CONGESTED;
> > > >  	if (bdi == current->backing_dev_info)
> > > > -		return 1;
> > > > +		return QUEUEWRITE_ALLOWED;
> > > >  
> > > >  	/* lumpy reclaim for hugepage often need a lot of write */
> > > >  	if (sc->order > PAGE_ALLOC_COSTLY_ORDER)
> > > > -		return 1;
> > > > -	return 0;
> > > > +		return QUEUEWRITE_ALLOWED;
> > > > +	return ret;
> > > >  }
> > > 
> > > The function can't return QUEUEXXX_DENIED.
> > > It can affect disable_lumpy_reclaim. 
> > > 
> > 
> > Yes, but that change was made in "vmscan: Narrow the scenarios lumpy
> > reclaim uses synchrounous reclaim". Maybe I am misunderstanding your
> > objection.
> 
> I means current may_write_to_queue never returns QUEUEWRITE_DENIED.
> What's the role of it?
> 

As of now, little point because QUEUEWRITE_CONGESTED implies denied. I was allowing
the possibility of distinguishing between these cases in the future depending
on what happened with wait_iff_congested(). I will drop it for simplicity
and reintroduce it when or if there is a distinction between
denied and congested.

> In addition, we don't need disable_lumpy_reclaim_mode() in pageout.
> That's because both PAGE_KEEP and PAGE_KEEP_CONGESTED go to keep_locked
> and calls disable_lumpy_reclaim_mode at last. 
> 

True, good spot.

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab