Re: [PATCH] mm/compaction: correct to flush migrated pages if pageblock skip happens

From: Vlastimil Babka
Date: Mon Aug 24 2015 - 04:51:40 EST

On 08/21/2015 05:56 AM, Joonsoo Kim wrote:
We cache isolate_start_pfn before entering isolate_migratepages().
If pageblock is skipped in isolate_migratepages() due to whatever reason,
cc->migrate_pfn could be far from isolate_start_pfn hence flushing pages
that were freed happens. For example, following scenario can be possible.

- assume order-9 compaction, pageblock order is 9
- start_isolate_pfn is 0x200
- isolate_migratepages()
- skip a number of pageblocks
- start to isolate from pfn 0x600
- cc->migrate_pfn = 0x620
- return
- last_migrated_pfn is set to 0x200
- check flushing condition
- current_block_start is set to 0x600
- last_migrated_pfn < current_block_start then do useless flush

This wrong flush would not help the performance and success rate so
this patch try to fix it. One simple way to know exact position
where we start to isolate migratable pages is that we cache it
in isolate_migratepages() before entering actual isolation. This patch
implements it and fix the problem.

Yeah, that should work.

Signed-off-by: Joonsoo Kim <>

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

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