linux-kernel - Re: [PATCH resend] mm: drain percpu lru add/rotate page-vectors on cpu hot-unplug

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Date:	Thu, 01 Mar 2012 12:06:46 +0400
From:	Konstantin Khlebnikov <khlebnikov@...nvz.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Johannes Weiner <jweiner@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Subject: Re: [PATCH resend] mm: drain percpu lru add/rotate page-vectors on
 cpu hot-unplug

Andrew Morton wrote:
> On Tue, 28 Feb 2012 23:40:45 +0400
> Konstantin Khlebnikov<khlebnikov@...nvz.org>  wrote:
>
>> This cpu hotplug hook was accidentally removed in commit v2.6.30-rc4-18-g00a62ce
>> ("mm: fix Committed_AS underflow on large NR_CPUS environment")
>
> That was a long time ago - maybe we should leave it removed ;) I mean,
> did this bug(?) have any visible effect?  If so, what was that effect?

It's because cpu hotplug/unplug isn't widely used feature.
Visible effect -- some pages are borrowed in per-cpu page-vectors.
Truncate can deal with it, but these pages cannot be reused while this cpu is offline.
So this is like temporary memory leak.

>
> IOW, the changelog didn't give anyone any reason to apply the patch to
> anything!

Sorry, I'm just stuck in pile of patches. It seems I should stop and send them one by one.
This one isn't critical, so there no reasons for pushing it into stable branches.
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