linux-kernel - Re: Accounting problem of MIGRATE

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Date:	Wed, 20 Jun 2012 22:45:17 -0400
From:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
To:	Minchan Kim <minchan@...nel.org>
Cc:	Aaditya Kumar <aaditya.kumar.30@...il.com>,
	Mel Gorman <mel@....ul.ie>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Accounting problem of MIGRATE_ISOLATED freed page

On Wed, Jun 20, 2012 at 9:55 PM, Minchan Kim <minchan@...nel.org> wrote:
> On 06/21/2012 10:39 AM, KOSAKI Motohiro wrote:
>
>>>> number of isolate page block is almost always 0. then if we have such counter,
>>>> we almost always can avoid zone->lock. Just idea.
>>>
>>> Yeb. I thought about it but unfortunately we can't have a counter for MIGRATE_ISOLATE.
>>> Because we have to tweak in page free path for pages which are going to free later after we
>>> mark pageblock type to MIGRATE_ISOLATE.
>>
>> I mean,
>>
>> if (nr_isolate_pageblock != 0)
>>    free_pages -= nr_isolated_free_pages(); // your counting logic
>>
>> return __zone_watermark_ok(z, alloc_order, mark,
>>                               classzone_idx, alloc_flags, free_pages);
>>
>>
>> I don't think this logic affect your race. zone_watermark_ok() is already
>> racy. then new little race is no big matter.
>
>
> It seems my explanation wasn't enough. :(
> I already understand your intention but we can't make nr_isolate_pageblock.
> Because we should count two type of free pages.

I mean, move_freepages_block increment number of page *block*, not pages.
number of free *pages* are counted by zone_watermark_ok_safe().


> 1. Already freed page so they are already in buddy list.
>   Of course, we can count it with return value of move_freepages_block(zone, page, MIGRATE_ISOLATE) easily.
>
> 2. Will be FREEed page by do_migrate_range.
>   It's a _PROBLEM_. For it, we should tweak free path. No?

No.


> If All of pages are PageLRU when hot-plug happens(ie, 2), nr_isolate_pagblock is zero and
> zone_watermk_ok_safe can't do his role.

number of isolate pageblock don't depend on number of free pages. It's
a concept of
an attribute of PFN range.
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