Re: [PATCH v6 11/13] mm, compaction: skip buddy pages by their order in the migrate scanner

[Vlastimil Babka]

On 08/22/2014 12:11 AM, Andrew Morton wrote:
> On Mon,  4 Aug 2014 10:55:22 +0200 Vlastimil Babka <vbabka@xxxxxxx> wrote:
>
> > The migration scanner skips PageBuddy pages, but does not consider their order
> > as checking page_order() is generally unsafe without holding the zone->lock,
> > and acquiring the lock just for the check wouldn't be a good tradeoff.
> >
> > Still, this could avoid some iterations over the rest of the buddy page, and
> > if we are careful, the race window between PageBuddy() check and page_order()
> > is small, and the worst thing that can happen is that we skip too much and miss
> > some isolation candidates. This is not that bad, as compaction can already fail
> > for many other reasons like parallel allocations, and those have much larger
> > race window.
> >
> > This patch therefore makes the migration scanner obtain the buddy page order
> > and use it to skip the whole buddy page, if the order appears to be in the
> > valid range.
> >
> > It's important that the page_order() is read only once, so that the value used
> > in the checks and in the pfn calculation is the same. But in theory the
> > compiler can replace the local variable by multiple inlines of page_order().
> > Therefore, the patch introduces page_order_unsafe() that uses ACCESS_ONCE to
> > prevent this.
> >
> > Testing with stress-highalloc from mmtests shows a 15% reduction in number of
> > pages scanned by migration scanner. The reduction is >60% with __GFP_NO_KSWAPD
> > allocations, along with success rates better by few percent.
> > This change is also a prerequisite for a later patch which is detecting when
> > a cc->order block of pages contains non-buddy pages that cannot be isolated,
> > and the scanner should thus skip to the next block immediately.
>
> What is this "later patch"?  Or is the changelog stale?

Yes it is stale, that later patch was postponed due to apparent bad 
effect on fragmentation. I guess we can drop the last paragraph from 
this commit log.
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