linux-kernel - Re: [PATCH] exit: clear TIF_MEMDIE after exit_task

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Date:	Mon, 13 Jun 2016 22:52:43 +0900
From:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:	mhocko@...nel.org, mst@...hat.com
Cc:	vdavydov@...tuozzo.com, akpm@...ux-foundation.org,
	rientjes@...gle.com, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] exit: clear TIF_MEMDIE after exit_task_work

Michal Hocko wrote:
> I have checked the vnet code and it doesn't seem to rely on
> copy_from_user/get_user AFAICS. Other users of use_mm() need to copy to
> the userspace only as well. So we should be perfectly safe to OOM reap
> address space even when it is shared by the kthread [1] so this is
> not really needed for the OOM correctness purpose. It would be much
> nicer if the kthread didn't pin the mm for two long outside of the OOM
> handling as well of course but that lowers the priority of the change.
> 
> [1] http://lkml.kernel.org/r/20160613112348.GC6518@dhcp22.suse.cz

It seems to me that vhost code relies on copy from the userspace.

use_mm(dev->mm) and unuse_mm(dev->mm) are used inside vhost_worker().
work->fn(work) is initialized by vhost_work_init().
vhost_scsi_open() passes vhost_scsi_complete_cmd_work() and
vhost_scsi_evt_work() as ->fn, and both functions call __get_user().

vhost_scsi_complete_cmd_work() {
  vhost_signal() {
    vhost_notify() {
      __get_user()
    }
  }
}

vhost_scsi_evt_work() {
  vhost_scsi_do_evt_work() {
    vhost_get_vq_desc() {
      __get_user() / __copy_from_user()
      get_indirect() {
        copy_from_iter()
      }
    }
  }
}