linux-kernel - Re: [PATCH] mm: compaction: Check pfn_valid when entering a new MAX_ORDER_NR_PAGES block during isolation for migration

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Date:	Tue, 31 Jan 2012 12:40:26 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Mel Gorman <mgorman@...e.de>
Cc:	Herbert van den Bergh <herbert.van.den.bergh@...cle.com>,
	Michal Nazarewicz <mina86@...a86.com>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: compaction: Check pfn_valid when entering a new
 MAX_ORDER_NR_PAGES block during isolation for migration

On Tue, 31 Jan 2012 16:35:28 +0000
Mel Gorman <mgorman@...e.de> wrote:

> When isolating for migration, migration starts at the start of a zone
> which is not necessarily pageblock aligned. Further, it stops isolating
> when COMPACT_CLUSTER_MAX pages are isolated so migrate_pfn is generally
> not aligned.
> 
> The problem is that pfn_valid is only called on the first PFN being
> checked. Lets say we have a case like this
> 
> H = MAX_ORDER_NR_PAGES boundary
> | = pageblock boundary
> m = cc->migrate_pfn
> f = cc->free_pfn
> o = memory hole
> 
> H------|------H------|----m-Hoooooo|ooooooH-f----|------H
> 
> The migrate_pfn is just below a memory hole and the free scanner is
> beyond the hole. When isolate_migratepages started, it scans from
> migrate_pfn to migrate_pfn+pageblock_nr_pages which is now in a memory
> hole. It checks pfn_valid() on the first PFN but then scans into the
> hole where there are not necessarily valid struct pages.
> 
> This patch ensures that isolate_migratepages calls pfn_valid when
> necessary.
> 
> Reported-and-tested-by: Herbert van den Bergh <herbert.van.den.bergh@...cle.com>
> Signed-off-by: Mel Gorman <mgorman@...e.de>
> Acked-by: Michal Nazarewicz <mina86@...a86.com>

The changelog forgot to describe the user-visible effects of the bug.

> Cc: stable@...nel.org

So he (and others) will be confused.
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