linux-kernel - Re: [PATCH] mm, oom: fix potential data corruption when oom

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Message-Id: <201708050149.JEC09861.MOLFtQFFVJOSOH@I-love.SAKURA.ne.jp>
Date:   Sat, 5 Aug 2017 01:49:17 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     mhocko@...nel.org
Cc:     linux-mm@...ck.org, akpm@...ux-foundation.org,
        wenwei.tww@...baba-inc.com, oleg@...hat.com, rientjes@...gle.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm, oom: fix potential data corruption when oom_reaper races with writer

Michal Hocko wrote:
> And that's why we still see the corruption. That, however, means that
> the MMF_UNSTABLE implementation has to be more complex and we have to
> hook into all anonymous memory fault paths which I hoped I could avoid
> previously.

I don't understand mm internals including pte/ptl etc. , but I guess that
the direction is correct. Since the OOM reaper basically does

  Set MMF_UNSTABLE flag on mm_struct.
  For each reapable page in mm_struct {
    Take ptl lock.
    Remove pte.
    Release ptl lock.
  }

the page fault handler will need to check MMF_UNSTABLE with lock held.

  For each faulted page in mm_struct {
    Take ptl lock.
    Add pte only if MMF_UNSTABLE flag is not set.
    Release ptl lock.
  }