Re: [PATCH] mm: __rmqueue_fallback() should respect pageblock type

[KOSAKI Motohiro]

(10/31/13 12:35 AM), Andrew Morton wrote:
> On Thu, 31 Oct 2013 00:24:49 -0400 kosaki.motohiro@xxxxxxxxx wrote:
>
> > When __rmqueue_fallback() don't find out a free block with the same size
> > of required, it splits a larger page and puts back rest peiece of the page
> > to free list.
> >
> > But it has one serious mistake. When putting back, __rmqueue_fallback()
> > always use start_migratetype if type is not CMA. However, __rmqueue_fallback()
> > is only called when all of start_migratetype queue are empty. That said,
> > __rmqueue_fallback always put back memory to wrong queue except
> > try_to_steal_freepages() changed pageblock type (i.e. requested size is
> > smaller than half of page block). Finally, antifragmentation framework
> > increase fragmenation instead of decrease.
> >
> > Mel's original anti fragmentation do the right thing. But commit 47118af076
> > (mm: mmzone: MIGRATE_CMA migration type added) broke it.
> >
> > This patch restores sane and old behavior.
>
> What are the user-visible runtime effects of this change?

Memory fragmentation may increase compaction rate. (And then system get unnecessary
slow down)


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