Re: [patch 1/4] mmu_notifier: Core code

Christoph Lameter
2008-01-25T19:03:39Z

On Fri, 25 Jan 2008, Robin Holt wrote:

> > > > +void mmu_notifier_release(struct mm_struct *mm)
> > > > +{
> > > > + struct mmu_notifier *mn;
> > > > + struct hlist_node *n;
> > > > +
> > > > + if (unlikely(!hlist_empty(&mm->mmu_notifier.head))) {
> > > > + rcu_read_lock();
> > > > + hlist_for_each_entry_rcu(mn, n,
> > > > + &mm->mmu_notifier.head, hlist) {
> > > > + if (mn->ops->release)
> > > > + mn->ops->release(mn, mm);
> > > > + hlist_del(&mn->hlist);
> > >
> > > I think the hlist_del needs to be before the function callout so we can free
> > > the structure without a use-after-free issue.
> >
> > The list head is in the mm_struct. This will be freed later.
> >
>
> I meant the structure pointed to by &mn. I assume it is intended that
> structure be kmalloc'd as part of a larger structure. The driver is the
> entity which created that structure and should be the one to free it.

mn will be pointing to the listhead in the mm_struct one after the other.
You mean the ops structure?

> > > > +void mmu_notifier_register(struct mmu_notifier *mn, struct mm_struct *mm)
> > > > +{
> > > > + spin_lock(&mmu_notifier_list_lock);
> > >
> > > Shouldn't this really be protected by the down_write(mmap_sem)? Maybe:
> >
> > Ok. We could switch this to mmap_sem protection for the mm_struct but the
> > rmap notifier is not associated with an mm_struct. So we would need to
> > keep it there. Since we already have a spinlock: Just use it for both to
> > avoid further complications.
>
> But now you are putting a global lock in where it is inappropriate.

The lock is only used during register and unregister. Very low level
usage.

> > > XPMEM, would also benefit from a call early. We could make all the
> > > segments as being torn down and start the recalls. We already have
> > > this code in and working (have since it was first written 6 years ago).
> > > In this case, all segments are torn down with a single message to each
> > > of the importing partitions. In contrast, the teardown code which would
> > > happen now would be one set of messages for each vma.
> >
> > So we need an additional global teardown call? Then we'd need to switch
> > off the vma based invalidate_range()?
>
> No, EXACTLY what I originally was asking for, either move this call site
> up, introduce an additional mmu_notifier op, or place this one in two
> locations with a flag indicating which call is being made.

Add a new invalidate_all() call? Then on exit we do

1. invalidate_all()

2. invalidate_range() for each vma

3. release()

We cannot simply move the call up because there will be future range
callbacks on vma invalidation.