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SubjectRe: [PATCH 2/7] mmu_notifier: keep track of active invalidation ranges v2
On Thu, Dec 25, 2014 at 10:29:44AM +0200, Haggai Eran wrote:
> On 22/12/2014 18:48, j.glisse@gmail.com wrote:
> > static inline void mmu_notifier_invalidate_range_start(struct mm_struct *mm,
> > - unsigned long start,
> > - unsigned long end,
> > - enum mmu_event event)
> > + struct mmu_notifier_range *range)
> > {
> > + /*
> > + * Initialize list no matter what in case a mmu_notifier register after
> > + * a range_start but before matching range_end.
> > + */
> > + INIT_LIST_HEAD(&range->list);
>
> I don't see how can an mmu_notifier register after a range_start but
> before a matching range_end. The mmu_notifier registration locks all mm
> locks, and that should prevent any invalidation from running, right?

File invalidation (like truncation) can lead to this case.

>
> > if (mm_has_notifiers(mm))
> > - __mmu_notifier_invalidate_range_start(mm, start, end, event);
> > + __mmu_notifier_invalidate_range_start(mm, range);
> > }
>
> ...
>
> > void __mmu_notifier_invalidate_range_start(struct mm_struct *mm,
> > - unsigned long start,
> > - unsigned long end,
> > - enum mmu_event event)
> > + struct mmu_notifier_range *range)
> >
> > {
> > struct mmu_notifier *mn;
> > @@ -185,21 +183,36 @@ void __mmu_notifier_invalidate_range_start(struct mm_struct *mm,
> > id = srcu_read_lock(&srcu);
> > hlist_for_each_entry_rcu(mn, &mm->mmu_notifier_mm->list, hlist) {
> > if (mn->ops->invalidate_range_start)
> > - mn->ops->invalidate_range_start(mn, mm, start,
> > - end, event);
> > + mn->ops->invalidate_range_start(mn, mm, range);
> > }
> > srcu_read_unlock(&srcu, id);
> > +
> > + /*
> > + * This must happen after the callback so that subsystem can block on
> > + * new invalidation range to synchronize itself.
> > + */
> > + spin_lock(&mm->mmu_notifier_mm->lock);
> > + list_add_tail(&range->list, &mm->mmu_notifier_mm->ranges);
> > + mm->mmu_notifier_mm->nranges++;
> > + spin_unlock(&mm->mmu_notifier_mm->lock);
> > }
> > EXPORT_SYMBOL_GPL(__mmu_notifier_invalidate_range_start);
>
> Don't you have a race here because you add the range struct after the
> callback?
>
> -------------------------------------------------------------------------
> Thread A | Thread B
> -------------------------------------------------------------------------
> call mmu notifier callback |
> clear SPTE |
> | device page fault
> | mmu_notifier_range_is_valid returns true
> | install new SPTE
> add event struct to list |
> mm clears/modifies the PTE |
> -------------------------------------------------------------------------
>
> So we are left with different entries in the host page table and the
> secondary page table.
>
> I would think you'd want the event struct to be added to the list before
> the callback is run.
>

Yes you right, but the comment i left trigger memory that i did that on
purpose a one point probably with a different synch mecanism inside hmm.
I will try to medidate a bit see if i can bring back memory why i did it
that way in respect to previous design.

In all case i will respin with that order modified. Can i add you review
by after doing so ?

Cheers,
Jérôme


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