Re: [RFC 12/13] mm, compaction: more reliably increase direct compaction priority

[Vlastimil Babka]

On 05/31/2016 02:07 PM, Vlastimil Babka wrote:
> On 05/31/2016 08:37 AM, Joonsoo Kim wrote:
> > > @@ -3695,22 +3695,22 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
> > >  	else
> > >  		no_progress_loops++;
> > >
> > > -	if (should_reclaim_retry(gfp_mask, order, ac, alloc_flags,
> > > -				 did_some_progress > 0, no_progress_loops))
> > > -		goto retry;
> > > -
> > > +	should_retry = should_reclaim_retry(gfp_mask, order, ac, alloc_flags,
> > > +				 did_some_progress > 0, no_progress_loops);
> > >  	/*
> > >  	 * It doesn't make any sense to retry for the compaction if the order-0
> > >  	 * reclaim is not able to make any progress because the current
> > >  	 * implementation of the compaction depends on the sufficient amount
> > >  	 * of free memory (see __compaction_suitable)
> > >  	 */
> > > -	if (did_some_progress > 0 &&
> > > -			should_compact_retry(ac, order, alloc_flags,
> > > +	if (did_some_progress > 0)
> > > +		should_retry |= should_compact_retry(ac, order, alloc_flags,
> > >  				compact_result, &compact_priority,
> > > -				compaction_retries))
> > > +				compaction_retries);
> > > +	if (should_retry)
> > >  		goto retry;
> >
> > Hmm... it looks odd that we check should_compact_retry() when
> > did_some_progress > 0. If system is full of anonymous memory and we
> > don't have swap, we can't reclaim anything but we can compact.
>
> Right, thanks.

Hmm on the other hand, should_compact_retry will assume (in 
compaction_zonelist_suitable()) that reclaimable memory is actually 
reclaimable. If there's nothing to tell us that it actually isn't, if we 
drop the reclaim progress requirement. That's risking an infinite loop?