linux-kernel - Re: [PATCH v3] memory-hotplug: fix zone stat mismatch

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Message-Id: <20120920144232.a3e8b60f.akpm@linux-foundation.org>
Date:	Thu, 20 Sep 2012 14:42:32 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Minchan Kim <minchan@...nel.org>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Subject: Re: [PATCH v3] memory-hotplug: fix zone stat mismatch

On Thu, 20 Sep 2012 15:43:25 +0900
Minchan Kim <minchan@...nel.org> wrote:

> During memory-hotplug, I found NR_ISOLATED_[ANON|FILE]
> are increasing so that kernel are hang out.
> 
> The cause is that when we do memory-hotadd after memory-remove,
> __zone_pcp_update clear out zone's ZONE_STAT_ITEMS in setup_pageset
> although vm_stat_diff of all CPU still have value.
> 
> In addtion, when we offline all pages of the zone, we reset them
> in zone_pcp_reset without drain so that we lost zone stat item.
> 

Here's what I ended up with for a changelog:

: During memory-hotplug, I found NR_ISOLATED_[ANON|FILE] are increasing,
: causing the kernel to hang.  When the system doesn't have enough free
: pages, it enters reclaim but never reclaim any pages due to
: too_many_isolated()==true and loops forever.
: 
: The cause is that when we do memory-hotadd after memory-remove,
: __zone_pcp_update() clears a zone's ZONE_STAT_ITEMS in setup_pageset()
: although the vm_stat_diff of all CPUs still have values.
: 
: In addtion, when we offline all pages of the zone, we reset them in
: zone_pcp_reset without draining so we loss some zone stat item.

As memory hotplug seems fairly immature and broken, I'm thinking
there's no point in backporting this into -stable.  And I don't *think*
we really need it in 3.6 either?  (It doesn't apply cleanly to current
mainline anyway - I didn't check why).

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