Re: [PATCH v2 0/4] fix premature OOM regression in 4.7+ due to cpuset races

[Hillf Danton]

On Friday, January 20, 2017 6:39 PM Vlastimil Babka wrote: 
> 
> Changes since v1:
> - add/remove comments per Michal Hocko and Hillf Danton
> - move no_zone: label in patch 3 so we don't miss part of ac initialization
> 
> This is v2 of my attempt to fix the recent report based on LTP cpuset stress
> test [1]. The intention is to go to stable 4.9 LTSS with this, as triggering
> repeated OOMs is not nice. That's why the patches try to be not too intrusive.
> 
> Unfortunately why investigating I found that modifying the testcase to use
> per-VMA policies instead of per-task policies will bring the OOM's back, but
> that seems to be much older and harder to fix problem. I have posted a RFC [2]
> but I believe that fixing the recent regressions has a higher priority.
> 
> Longer-term we might try to think how to fix the cpuset mess in a better and
> less error prone way. I was for example very surprised to learn, that cpuset
> updates change not only task->mems_allowed, but also nodemask of mempolicies.
> Until now I expected the parameter to alloc_pages_nodemask() to be stable.
> I wonder why do we then treat cpusets specially in get_page_from_freelist()
> and distinguish HARDWALL etc, when there's unconditional intersection between
> mempolicy and cpuset. I would expect the nodemask adjustment for saving
> overhead in g_p_f(), but that clearly doesn't happen in the current form.
> So we have both crazy complexity and overhead, AFAICS.
> 
> [1] https://lkml.kernel.org/r/CAFpQJXUq-JuEP=QPidy4p_=FN0rkH5Z-kfB4qBvsf6jMS87Edg@xxxxxxxxxxxxxx
> [2] https://lkml.kernel.org/r/7c459f26-13a6-a817-e508-b65b903a8378@xxxxxxx
> 
> Vlastimil Babka (4):
>   mm, page_alloc: fix check for NULL preferred_zone
>   mm, page_alloc: fix fast-path race with cpuset update or removal
>   mm, page_alloc: move cpuset seqcount checking to slowpath
>   mm, page_alloc: fix premature OOM when racing with cpuset mems update
> 
>  include/linux/mmzone.h |  6 ++++-
>  mm/page_alloc.c        | 68 ++++++++++++++++++++++++++++++++++----------------
>  2 files changed, 52 insertions(+), 22 deletions(-)
> 
Acked-by: Hillf Danton <hillf.zj@xxxxxxxxxxxxxxx>