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

Christoph Lameter
2008-01-25T18:47:18Z

On Fri, 25 Jan 2008, Robin Holt wrote:

> I realize it is a minor nit, but since we put the continuation in column
> 81 in the next define, can we do the same here and make this more
> readable?

We need to fix the next define to not use column 81.
Found a couple of more 80 column infractions. Will be fixed in next
release.

> > +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.

> > +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.

> > + spin_lock(&mmu_notifier_list_lock);
> > + hlist_del(&mn->hlist);
>
> hlist_del_rcu? Ditto on the lock.

Peter already mentioned that and I have posted patches that address this
issue.

> > @@ -2043,6 +2044,7 @@ void exit_mmap(struct mm_struct *mm)
> > vm_unacct_memory(nr_accounted);
> > free_pgtables(&tlb, vma, FIRST_USER_ADDRESS, 0);
> > tlb_finish_mmu(tlb, 0, end);
> > + mmu_notifier_release(mm);
>
> Can we consider moving this notifier or introducing an additional notifier
> in the release or a flag to this one indicating early/late.

There is only one call right now?

> The GRU that Jack is concerned with would benefit from the early in
> that it could just invalidate the GRU context and immediately all GRU
> TLB entries are invalid. I believe Jack would like to also be able to
> remove his entry from the mmu_notifier list in an effort to avoid the
> page and range callouts.

The TLB entries are removed by earlier invalidate_range calls. I would
think that no TLBs are left at this point. Its simply a matter of
releasing any still allocated resources through this callback.

> 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()?