Re: [RFC v2] memory-hotplug: remove MIGRATE_ISOLATE fromfree_area->free_list

[Minchan Kim]

Hi Bart,

On Thu, Sep 06, 2012 at 06:34:35PM +0200, Bartlomiej Zolnierkiewicz wrote:
> 
> Hi,
> 
> On Thursday 06 September 2012 04:53:38 Minchan Kim wrote:
> > Normally, MIGRATE_ISOLATE type is used for memory-hotplug.
> > But it's irony type because the pages isolated would exist
> > as free page in free_area->free_list[MIGRATE_ISOLATE] so people
> > can think of it as allocatable pages but it is *never* allocatable.
> > It ends up confusing NR_FREE_PAGES vmstat so it would be
> > totally not accurate so some of place which depend on such vmstat
> > could reach wrong decision by the context.
> > 
> > There were already report about it.[1]
> > [1] 702d1a6e, memory-hotplug: fix kswapd looping forever problem
> > 
> > Then, there was other report which is other problem.[2]
> > [2] http://www.spinics.net/lists/linux-mm/msg41251.html
> > 
> > I believe it can make problems in future, too.
> > So I hope removing such irony type by another design.
> > 
> > I hope this patch solves it and let's revert [1] and doesn't need [2].
> > 
> > * Changelog v1
> >  * Fix from Michal's many suggestion
> > 
> > Cc: Michal Nazarewicz <mina86@xxxxxxxxxx>
> > Cc: Mel Gorman <mel@xxxxxxxxx>
> > Cc: Kamezawa Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
> > Cc: Yasuaki Ishimatsu <isimatu.yasuaki@xxxxxxxxxxxxxx>
> > Cc: Wen Congyang <wency@xxxxxxxxxxxxxx>
> > Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> > Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
> > ---
> > It's very early version which show the concept so I still marked it with RFC.
> > I just tested it with simple test and works.
> > This patch is needed indepth review from memory-hotplug guys from fujitsu
> > because I saw there are lots of patches recenlty they sent to about
> > memory-hotplug change. Please take a look at this patch.
> 
> [...]
> 
> > @@ -948,8 +954,13 @@ static int move_freepages(struct zone *zone,
> >  		}
> >  
> >  		order = page_order(page);
> > -		list_move(&page->lru,
> > -			  &zone->free_area[order].free_list[migratetype]);
> > +		if (migratetype != MIGRATE_ISOLATE) {
> > +			list_move(&page->lru,
> > +				&zone->free_area[order].free_list[migratetype]);
> > +		} else {
> > +			list_del(&page->lru);
> > +			isolate_free_page(page, order);
> > +		}
> >  		page += 1 << order;
> >  		pages_moved += 1 << order;
> >  	}
> 
> Shouldn't NR_FREE_PAGES counter be decreased somewhere above?
> 
> [ I can see that it is not modified in __free_pages_ok() and
>   free_hot_cold_page() because page is still counted as non-free one but
>   here situation is different AFAICS. ]
> 
> I tested the patch locally here with CONFIG_CMA=y and it causes some
> major problems for CMA (multiple errors from dma_alloc_from_contiguous()
> about memory ranges being busy and allocation failures).
> 
> [ I'm sorry that I don't know more details yet but the issue should be
>   easily reproducible. ]

At the moment, I don't have a time to look into that so I will revisit
in near future. 
Thanks for the review and test!

> 
> Best regards,
> --
> Bartlomiej Zolnierkiewicz
> Samsung Poland R&D Center
> 
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-- 
Kind regards,
Minchan Kim
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