On Fri, Nov 18, 2011 at 05:43:10PM +0100, Marek Szyprowski wrote:

quoted

From: Michal Nazarewicz <redacted>

diff --git a/mm/compaction.c b/mm/compaction.c
index 6afae0e..09c9702 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c

@@ -111,7 +111,10 @@ skip:

 next:
 		pfn += isolated;
-		page += isolated;
+		if (zone_pfn_same_memmap(pfn - isolated, pfn))
+			page += isolated;
+		else
+			page = pfn_to_page(pfn);
 	}

On Mon, 12 Dec 2011 15:19:53 +0100, Mel Gorman [off-list ref] wrote:

Is this necessary?

We are isolating pages, the largest of which is a MAX_ORDER_NR_PAGES
page.  [...]

On Mon, 12 Dec 2011 15:40:30 +0100, Mel Gorman [off-list ref] wrote:

To be clear, I'm referring to a single page being isolated here. It may
or may not be a high-order page but it's still going to be less then
MAX_ORDER_NR_PAGES so you should be able check when a new block is
entered and pfn_to_page is necessary.

Do you mean something like:

if (same pageblock)
	just do arithmetic;
else
	use pfn_to_page;

?

I've discussed it with Dave and he suggested that approach as an
optimisation since in some configurations zone_pfn_same_memmap()
is always true thus compiler will strip the else part, whereas
same pageblock test will be false on occasions regardless of kernel
configuration.

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