Re: [PATCH mmotm 3/3] mm, compaction: prevent nr_isolated_* from going negative

[Vlastimil Babka]

On 04/26/2016 02:55 AM, Joonsoo Kim wrote:
> On Mon, Apr 25, 2016 at 03:35:50PM +0200, Vlastimil Babka wrote:
> > @@ -846,9 +845,11 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
> >  				spin_unlock_irqrestore(&zone->lru_lock,	flags);
> >  				locked = false;
> >  			}
> > -			putback_movable_pages(migratelist);
> > -			nr_isolated = 0;
> > +			acct_isolated(zone, cc);
> > +			putback_movable_pages(&cc->migratepages);
> > +			cc->nr_migratepages = 0;
> >  			cc->last_migrated_pfn = 0;
> > +			nr_isolated = 0;
>
> Is it better to use separate list and merge it cc->migratepages when
> finishing instead of using cc->migratepages directly? If
> isolate_migratepages() try to isolate more than one page block and keep
> isolated page on previous pageblock, this putback all will invalidate
> all the previous work. It would be beyond of the scope of this
> function. Now, isolate_migratepages() try to isolate the page in one
> pageblock so this code is safe. But, I think that removing such
> dependency will be helpful in the future. I'm not strongly insisting it
> so if you think it's not useful thing, please ignore this comment.

migratelist was merely a reference to cc->migratepages, so it wouldn't prevent 
the situation you are suggesting. A truly separate list would need to be 
appended to cc->migratepages when leaving isolate_migratepages_block() and 
there's no need to do that right now.

BTW, can you check patch 1/3? Thanks!

Vlastimil

> Thanks.