Re: [RFC PATCH v3 4/4] mm/page_alloc: restrict max order of merging on isolated pageblock

[Joonsoo Kim <iamjoonsoo.kim@lge.com>]

On Fri, Aug 29, 2014 at 12:52:44PM -0400, Naoya Horiguchi wrote:
> Hi Joonsoo,
> 
> On Tue, Aug 26, 2014 at 05:08:18PM +0900, Joonsoo Kim wrote:
> > Current pageblock isolation logic could isolate each pageblock
> > individually. This causes freepage accounting problem if freepage with
> > pageblock order on isolate pageblock is merged with other freepage on
> > normal pageblock. We can prevent merging by restricting max order of
> > merging to pageblock order if freepage is on isolate pageblock.
> > 
> > Side-effect of this change is that there could be non-merged buddy
> > freepage even if finishing pageblock isolation, because undoing pageblock
> > isolation is just to move freepage from isolate buddy list to normal buddy
> > list rather than to consider merging. But, I think it doesn't matter
> > because 1) almost allocation request are for equal or below pageblock
> > order, 2) caller of pageblock isolation will use this freepage so
> > freepage will split in any case and 3) merge would happen soon after
> > some alloc/free on this and buddy pageblock.
> > 
> > Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> > ---
> >  mm/page_alloc.c |   15 ++++++++++++---
> >  1 file changed, 12 insertions(+), 3 deletions(-)
> > 
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 809bfd3..8ba9fb0 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -570,6 +570,7 @@ static inline void __free_one_page(struct page *page,
> >  	unsigned long combined_idx;
> >  	unsigned long uninitialized_var(buddy_idx);
> >  	struct page *buddy;
> > +	int max_order = MAX_ORDER;
> >  
> >  	VM_BUG_ON(!zone_is_initialized(zone));
> >  
> > @@ -580,18 +581,26 @@ static inline void __free_one_page(struct page *page,
> >  	VM_BUG_ON(migratetype == -1);
> >  	if (unlikely(has_isolate_pageblock(zone))) {
> >  		migratetype = get_pfnblock_migratetype(page, pfn);
> > -		if (is_migrate_isolate(migratetype))
> > +		if (is_migrate_isolate(migratetype)) {
> > +			/*
> > +			 * We restrict max order of merging to prevent merge
> > +			 * between freepages on isolate pageblock and normal
> > +			 * pageblock. Without this, pageblock isolation
> > +			 * could cause incorrect freepage accounting.
> > +			 */
> > +			max_order = pageblock_order + 1;
> 
> When pageblock_order >= max_order, order in the while loop below could
> go beyond MAX_ORDER - 1. Or does it never happen?

Yes, you are right. Will fix it in next spin.

Thanks.

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