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

[Cody P Schafer]

On 04/10/2013 02:23 PM, Andrew Morton wrote:
> On Wed, 10 Apr 2013 11:23:28 -0700 Cody P Schafer <cody@xxxxxxxxxxxxxxxxxx> 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?

No, all the bugs are theoretical. Waiting should be fine.

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