Re: [PATCH -mm v2] mm: have order > 0 compaction start off whereit left

From: Rik van Riel
Date: Tue Jul 03 2012 - 11:00:21 EST

On 06/28/2012 07:27 PM, Minchan Kim wrote:

index 7ea259d..2668b77 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -422,6 +422,17 @@ static void isolate_freepages(struct zone *zone,
pfn -= pageblock_nr_pages) {
unsigned long isolated;

+ /*
+ * Skip ahead if another thread is compacting in the area
+ * simultaneously. If we wrapped around, we can only skip
+ * ahead if zone->compact_cached_free_pfn also wrapped to
+ * above our starting point.
+ */
+ if (cc->order> 0&& (!cc->wrapped ||

So if (partial_compaction(cc)&& ... ) or if (!full_compaction(cc)&& ...

I am not sure that we want to abstract away what is happening
here. We also are quite explicit with the meaning of cc->order
in compact_finished and other places in the compaction code.

+ zone->compact_cached_free_pfn>
+ cc->start_free_pfn))
+ pfn = min(pfn, zone->compact_cached_free_pfn);

The pfn can be where migrate_pfn below?
I mean we need this?

if (pfn<= low_pfn)
goto out;

That is a good point. I guess there is a small possibility that
another compaction thread is below us with cc->free_pfn and
cc->migrate_pfn, and we just inherited its cc->free_pfn via
zone->compact_cached_free_pfn, bringing us to below our own

Given that this was already possible with parallel compaction
in the past, I am not sure how important it is. It could result
in wasting a little bit of CPU, but your fix for it looks easy

Mel, any downside to compaction bailing (well, wrapping around)
a little earlier, like Minchan suggested?

@@ -463,6 +474,8 @@ static void isolate_freepages(struct zone *zone,
if (isolated)
high_pfn = max(high_pfn, pfn);
+ if (cc->order> 0)
+ zone->compact_cached_free_pfn = high_pfn;

Why do we cache high_pfn instead of pfn?

Reading the code, because we may not have isolated every
possible free page from this memory block. The same reason
cc->free_pfn is set to high_pfn right before the function

If we can't isolate any page, compact_cached_free_pfn would become low_pfn.
I expect it's not what you want.

I guess we should only cache the value of high_pfn if
we isolated some pages? In other words, this:

if (isolated) {
high_pfn = max(high_pfn, pfn);
if (cc->order > 0)
zone->compact_cached_free_pfn = high_pfn;

All rights reversed
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at