Messages in this thread | | | Subject | Re: [PATCH] Export mm_update_next_owner function for vhost-net | From | Jason Wang <> | Date | Thu, 13 Dec 2018 14:30:48 +0800 |
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On 2018/12/13 下午12:47, gchen.guomin@gmail.com wrote: > From: guomin chen <gchen.guomin@gmail.com> > > Under normal circumstances,When do_exit exits, mm->owner will > be updated on exit_mm(). but when the kernel process calls > unuse_mm() and then exits,mm->owner cannot be updated. And it > will point to a task that has been released. > > Below is my issue on vhost_net: > A, B are two kernel processes(such as vhost_worker), > C is a user space process(such as qemu), and all > three use the mm of the user process C. > Now, because user process C exits abnormally, the owner of this > mm becomes A. When A calls unuse_mm and exits, this mm->ower > still points to the A that has been released. > When B accesses this mm->owner again, A has been released.
Could you describe how you reproduce this issue? I believe vhost process should exit before process C?
> > Process A Process B > vhost_worker() vhost_worker() > --------- --------- > use_mm() use_mm() > ... > unuse_mm() > tsk->mm=NULL > do_exit() page fault > exit_mm() access mm->owner > can't update owner kernel Oops > > unuse_mm() > > Cc: "Michael S. Tsirkin" <mst@redhat.com> > Cc: Jason Wang <jasowang@redhat.com> > Cc: kvm@vger.kernel.org > Cc: virtualization@lists.linux-foundation.org > Cc: netdev@vger.kernel.org > Cc: linux-kernel@vger.kernel.org > Cc: "Eric W. Biederman" <ebiederm@xmission.com> > Cc: Andrew Morton <akpm@linux-foundation.org> > Cc: Sudip Mukherjee <sudipm.mukherjee@gmail.com> > Cc: "Luis R. Rodriguez" <mcgrof@kernel.org> > Cc: Dominik Brodowski <linux@dominikbrodowski.net> > Signed-off-by: guomin chen <gchen.guomin@gmail.com> > --- > drivers/vhost/vhost.c | 1 + > kernel/exit.c | 1 + > 2 files changed, 2 insertions(+) > > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c > index 6b98d8e..7c09087 100644 > --- a/drivers/vhost/vhost.c > +++ b/drivers/vhost/vhost.c > @@ -368,6 +368,7 @@ static int vhost_worker(void *data) > } > } > unuse_mm(dev->mm); > + mm_update_next_owner(dev->mm);
If you analysis is correct, this is still racy isn't it? (E.g page fault happen between unuse_mm() and mm_update_next_owner()).
Thanks
> set_fs(oldfs); > return 0; > } > diff --git a/kernel/exit.c b/kernel/exit.c > index 0e21e6d..9e046dd 100644 > --- a/kernel/exit.c > +++ b/kernel/exit.c > @@ -486,6 +486,7 @@ void mm_update_next_owner(struct mm_struct *mm) > task_unlock(c); > put_task_struct(c); > } > +EXPORT_SYMBOL(mm_update_next_owner); > #endif /* CONFIG_MEMCG */ > > /*
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