Re: [PATCH] oom_reaper: close race without using oom_lock

[Tetsuo Handa]

Michal Hocko wrote:
> On Wed 19-07-17 05:51:03, Tetsuo Handa wrote:
> > Michal Hocko wrote:
> > > On Tue 18-07-17 23:06:50, Tetsuo Handa wrote:
> > > > Commit e2fe14564d3316d1 ("oom_reaper: close race with exiting task")
> > > > guarded whole OOM reaping operations using oom_lock. But there was no
> > > > need to guard whole operations. We needed to guard only setting of
> > > > MMF_OOM_REAPED flag because get_page_from_freelist() in
> > > > __alloc_pages_may_oom() is called with oom_lock held.
> > > > 
> > > > If we change to guard only setting of MMF_OOM_SKIP flag, the OOM reaper
> > > > can start reaping operations as soon as wake_oom_reaper() is called.
> > > > But since setting of MMF_OOM_SKIP flag at __mmput() is not guarded with
> > > > oom_lock, guarding only the OOM reaper side is not sufficient.
> > > > 
> > > > If we change the OOM killer side to ignore MMF_OOM_SKIP flag once,
> > > > there is no need to guard setting of MMF_OOM_SKIP flag, and we can
> > > > guarantee a chance to call get_page_from_freelist() in
> > > > __alloc_pages_may_oom() without depending on oom_lock serialization.
> > > > 
> > > > This patch makes MMF_OOM_SKIP act as if MMF_OOM_REAPED, and adds a new
> > > > flag which acts as if MMF_OOM_SKIP, in order to close both race window
> > > > (the OOM reaper side and __mmput() side) without using oom_lock.
> > > 
> > > Why do we need this patch when
> > > http://lkml.kernel.org/r/20170626130346.26314-1-mhocko@xxxxxxxxxx
> > > already removes the lock and solves another problem at once?
> > 
> > We haven't got an answer from Hugh and/or Andrea whether that patch is safe.
> 
> So what? I haven't see anybody disputing the correctness. And to be
> honest I really dislike your patch. Yet another round kind of solutions
> are just very ugly hacks usually because they are highly timing
> sensitive.

Yes, OOM killer is highly timing sensitive.

> 
> > Even if that patch is safe, this patch still helps with CONFIG_MMU=n case.
> 
> Could you explain how?

Nothing prevents sequence below.

    Process-1              Process-2

    Takes oom_lock.
    Fails get_page_from_freelist().
    Enters out_of_memory().
    Gets SIGKILL.
    Gets TIF_MEMDIE.
    Leaves out_of_memory().
    Releases oom_lock.
    Enters do_exit().
    Calls __mmput().
                           Takes oom_lock.
                           Fails get_page_from_freelist().
    Releases some memory.
    Sets MMF_OOM_SKIP.
                           Enters out_of_memory().
                           Selects next victim because there is no !MMF_OOM_SKIP mm.
                           Sends SIGKILL needlessly.

If we ignore MMF_OOM_SKIP once, we can avoid sequence above.

    Process-1              Process-2

    Takes oom_lock.
    Fails get_page_from_freelist().
    Enters out_of_memory().
    Get SIGKILL.
    Get TIF_MEMDIE.
    Leaves out_of_memory().
    Releases oom_lock.
    Enters do_exit().
    Calls __mmput().
                           Takes oom_lock.
                           Fails get_page_from_freelist().
    Releases some memory.
    Sets MMF_OOM_SKIP.
                           Enters out_of_memory().
                           Ignores MMF_OOM_SKIP mm once.
                           Leaves out_of_memory().
                           Releases oom_lock.
                           Succeeds get_page_from_freelist().

Strictly speaking, this patch is independent with OOM reaper.
This patch increases possibility of succeeding get_page_from_freelist()
without sending SIGKILL. Your patch is trying to drop it silently.

Serializing setting of MMF_OOM_SKIP with oom_lock is one approach,
and ignoring MMF_OOM_SKIP once without oom_lock is another approach.