linux-kernel - Re: [PATCH v3 00/11] mm: fixup changers of per cpu pageset's ->high and ->batch

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Date:	Wed, 10 Apr 2013 14:23:54 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Cody P Schafer <cody@...ux.vnet.ibm.com>
Cc:	Gilad Ben-Yossef <gilad@...yossef.com>,
	Simon Jeons <simon.jeons@...il.com>,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	Mel Gorman <mgorman@...e.de>, Linux MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 00/11] mm: fixup changers of per cpu pageset's ->high
 and ->batch

On Wed, 10 Apr 2013 11:23:28 -0700 Cody P Schafer <cody@...ux.vnet.ibm.com> wrote:

> "Problems" with the current code:
>  1. there is a lack of synchronization in setting ->high and ->batch in
>     percpu_pagelist_fraction_sysctl_handler()
>  2. stop_machine() in zone_pcp_update() is unnecissary.
>  3. zone_pcp_update() does not consider the case where percpu_pagelist_fraction is non-zero
> 
> To fix:
>  1. add memory barriers, a safe ->batch value, an update side mutex when
>     updating ->high and ->batch, and use ACCESS_ONCE() for ->batch users that
>     expect a stable value.
>  2. avoid draining pages in zone_pcp_update(), rely upon the memory barriers added to fix #1
>  3. factor out quite a few functions, and then call the appropriate one.
> 
> Note that it results in a change to the behavior of zone_pcp_update(), which is
> used by memory_hotplug. I'm rather certain that I've diserned (and preserved)
> the essential behavior (changing ->high and ->batch), and only eliminated
> unneeded actions (draining the per cpu pages), but this may not be the case.
> 
> Further note that the draining of pages that previously took place in
> zone_pcp_update() occured after repeated draining when attempting to offline a
> page, and after the offline has "succeeded". It appears that the draining was
> added to zone_pcp_update() to avoid refactoring setup_pageset() into 2
> funtions.

There hasn't been a ton of review activity for this patchset :(

I'm inclined to duck it until after 3.9.  Do the patches fix any
noticeably bad userspace behavior?

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