Re: [PATCH 2/2] mm,oom: Do oom_task_origin() test in oom_badness().

[Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>]

Michal Hocko wrote:
> On Sat 21-05-16 11:01:30, Tetsuo Handa wrote:
> > Currently, oom_scan_process_thread() returns OOM_SCAN_SELECT if
> > oom_task_origin() returned true. But this might cause OOM livelock.
> > 
> > If the OOM killer finds a task with oom_task_origin(task) == true,
> > it means that that task is either inside try_to_unuse() from swapoff
> > path or unmerge_and_remove_all_rmap_items() from ksm's run_store path.
> > 
> > Let's take a look at try_to_unuse() as an example. Although there is
> > signal_pending() test inside the iteration loop, there are operations
> > (e.g. mmput(), wait_on_page_*()) which might block in unkillable state
> > waiting for other threads which might allocate memory.
> > 
> > Therefore, sending SIGKILL to a task with oom_task_origin(task) == true
> > can not guarantee that that task shall not stuck at unkillable waits.
> > Once the OOM reaper reaped that task's memory (or gave up reaping it),
> > the OOM killer must not select that task again when oom_task_origin(task)
> > returned true. We need to select different victims until that task can
> > hit signal_pending() test or finish the iteration loop.
> > 
> > Since oom_badness() is a function which returns score of the given thread
> > group with eligibility/livelock test, it is more natural and safer to let
> > oom_badness() return highest score when oom_task_origin(task) == true.
> > 
> > This patch moves oom_task_origin() test from oom_scan_process_thread() to
> > after MMF_OOM_REAPED test inside oom_badness(), changes the callers to
> > receive the score using "unsigned long" variable, and eliminates
> > OOM_SCAN_SELECT path in the callers.
> 
> I do not think this is the right approach. If the problem is real then
> the patch just papers over deficiency of the oom_task_origin which
> should be addressed instead.
> 
> > Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
> 
> Nacked-by: Michal Hocko <mhocko@suse.com>
> 

Quoting from http://lkml.kernel.org/r/20160523075524.GG2278@dhcp22.suse.cz :
> > Is it guaranteed that try_to_unuse() from swapoff is never blocked on memory
> > allocation (e.g. mmput(), wait_on_page_*()) ?
> 
> It shouldn't. All the waiting should be killable. If not it is a bug and
> should be fixed.

So, you think that we should replace mmput() with mmput_async(), lock_page()
with lock_page_killable(), wait_on_page_bit() with wait_on_page_bit_killable(),
mutex_lock() with mutex_lock_killable(), down_read() with down_read_killable()
and so on, don't you?

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