Re: [RFC PATCH v2 3/3] mm, oom: hand over MMF_OOM_SKIP to exit path if it is guranteed to finish

[Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>]

On 2018/10/30 21:10, Michal Hocko wrote:
> I misunderstood your concern. oom_reaper would back off without
> MMF_OOF_SKIP as well. You are right we cannot assume anything about
> close callbacks so MMF_OOM_SKIP has to come before that. I will move it
> behind the pagetable freeing.
> 

And at that point, your patch can at best wait for only __free_pgtables(),
at the cost/risk of complicating exit_mmap() and arch specific code. Also,
you are asking for comments to wrong audiences. It is arch maintainers who
need to precisely understand the OOM behavior / possibility of OOM lockup,
and you must persuade them about restricting/complicating future changes in
their arch code due to your wish to allow handover. Without "up-to-dated
big fat comments to all relevant functions affected by your change" and
"acks from all arch maintainers", I'm sure that people keep making
errors/mistakes/overlooks.

My patch can wait for completion of (not only exit_mmap() but also) __mmput(),
by using simple polling approach. My patch can allow NOMMU kernels to avoid
possibility of OOM lockup by setting MMF_OOM_SKIP at __mmput() (and future
patch will implement timeout based back off for NOMMU kernels), and allows you
to get rid of TIF_MEMDIE (which you recently added to your TODO list) by getting
rid of conditional handling of oom_reserves_allowed() and ALLOC_OOM.

Your "refusing timeout based next OOM victim selection" keeps everyone unable
to safely make forward progress. OOM handling is too much complicated, and
nobody can become free from errors/mistakes/overlooks. Look at the reality!