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

From: KOSAKI Motohiro
Date: Thu Oct 31 2013 - 01:00:55 EST

(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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