Re: [PATCH] exit: clear TIF_MEMDIE after exit_task_work

Michal Hocko
2016-06-07T12:50:19Z

On Mon 14-03-16 17:39:43, Michal Hocko wrote:
> On Tue 01-03-16 19:20:24, Michael S. Tsirkin wrote:
> > On Tue, Mar 01, 2016 at 06:17:58PM +0100, Michal Hocko wrote:
> [...]
> > > Sorry, I could have been more verbose... The code would have to make sure
> > > that the mm is still alive before calling g-u-p by
> > > atomic_inc_not_zero(&mm->mm_users) and fail if the user count dropped to
> > > 0 in the mean time. See how fs/proc/task_mmu.c does that (proc_mem_open
> > > + m_start + m_stop.
> > >
> > > The biggest advanatage would be that the mm address space pin would be
> > > only for the particular operation. Not sure whether that is possible in
> > > the driver though. Anyway pinning the mm for a potentially unbounded
> > > amount of time doesn't sound too nice.
> >
> > Hmm that would be another atomic on data path ...
> > I'd have to explore that.
>
> Did you have any chance to look into this?

So this is my take to get rid of mm_users pinning for an unbounded
amount of time. This is even not compile tested. I am not sure how to
handle when the mm goes away while there are still work items pending.
It seems this is not handled current anyway and only shouts with a
warning so this shouldn't cause a new regression AFAICS. I am not
familiar with the vnet code at all so I might be missing many things,
though. Does the below sound even remotely reasonable to you Michael?
---
diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index 669fef1e2bb6..47a3e2c832ea 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -343,7 +343,12 @@ static int vhost_worker(void *data)

if (work) {
__set_current_state(TASK_RUNNING);
+ if (!mmget_not_zero(dev->mm)) {
+ pr_warn("vhost: device owner mm got released unexpectedly\n");
+ break;
+ }
work->fn(work);
+ mmput(dev->mm);
if (need_resched())
schedule();
} else
@@ -481,7 +486,16 @@ long vhost_dev_set_owner(struct vhost_dev *dev)
}

/* No owner, become one */
- dev->mm = get_task_mm(current);
+ task_lock(current);
+ if (current->mm) {
+ dev->mm = current->mm;
+ atomic_inc(&curent->mm->mm_count);
+ }
+ task_unlock(current);
+ if (!dev->mm) {
+ err = -EINVAL;
+ goto err_mm;
+ }
worker = kthread_create(vhost_worker, dev, "vhost-%d", current->pid);
if (IS_ERR(worker)) {
err = PTR_ERR(worker);
@@ -505,7 +519,7 @@ long vhost_dev_set_owner(struct vhost_dev *dev)
dev->worker = NULL;
err_worker:
if (dev->mm)
- mmput(dev->mm);
+ mmdrop(dev->mm);
dev->mm = NULL;
err_mm:
return err;
@@ -583,7 +597,7 @@ void vhost_dev_cleanup(struct vhost_dev *dev, bool locked)
dev->worker = NULL;
}
if (dev->mm)
- mmput(dev->mm);
+ mmdrop(dev->mm);
dev->mm = NULL;
}
EXPORT_SYMBOL_GPL(vhost_dev_cleanup);
--
Michal Hocko
SUSE Labs