Re: [PATCH] Initialise mmu_notifier_range correctly

Jerome Glisse
2019-01-03T14:59:56Z

On Thu, Jan 03, 2019 at 06:39:08AM -0800, Matthew Wilcox wrote:
> On Thu, Jan 03, 2019 at 09:29:59AM -0500, Jerome Glisse wrote:
> > On Wed, Jan 02, 2019 at 08:18:33PM -0800, Matthew Wilcox wrote:
> > > On Wed, Jan 02, 2019 at 07:32:08PM -0800, John Hubbard wrote:
> > > > Having the range struct declared in separate places from the mmu_notifier_range_init()
> > > > calls is not great. But I'm not sure I see a way to make it significantly cleaner, given
> > > > that __follow_pte_pmd uses the range pointer as a way to decide to issue the mmn calls.
> > >
> > > Yeah, I don't think there's anything we can do. But I started reviewing
> > > the comments, and they don't make sense together:
> > >
> > > /*
> > > * Note because we provide range to follow_pte_pmd it will
> > > * call mmu_notifier_invalidate_range_start() on our behalf
> > > * before taking any lock.
> > > */
> > > if (follow_pte_pmd(vma->vm_mm, address, &range,
> > > &ptep, &pmdp, &ptl))
> > > continue;
> > >
> > > /*
> > > * No need to call mmu_notifier_invalidate_range() as we are
> > > * downgrading page table protection not changing it to point
> > > * to a new page.
> > > *
> > > * See Documentation/vm/mmu_notifier.rst
> > > */
> > >
> > > So if we don't call mmu_notifier_invalidate_range, why are we calling
> > > mmu_notifier_invalidate_range_start and mmu_notifier_invalidate_range_end?
> > > ie, why not this ...
> >
> > Thus comments looks wrong to me ... we need to call
> > mmu_notifier_invalidate_range() those are use by
> > IOMMU. I might be to blame for those comments thought.
>
> Yes, you're to blame for both of them.
>
> a4d1a88525138 (Jérôme Glisse 2017-08-31 17:17:26 -0400 791) * Note because we provide start/end to follow_pte_pmd it will
> a4d1a88525138 (Jérôme Glisse 2017-08-31 17:17:26 -0400 792) * call mmu_notifier_invalidate_range_start() on our behalf
> a4d1a88525138 (Jérôme Glisse 2017-08-31 17:17:26 -0400 793) * before taking any lock.
>
> 0f10851ea475e (Jérôme Glisse 2017-11-15 17:34:07 -0800 794) * No need to call mmu_notifier_invalidate_range() as we are
> 0f10851ea475e (Jérôme Glisse 2017-11-15 17:34:07 -0800 795) * downgrading page table protection not changing it to point
> 0f10851ea475e (Jérôme Glisse 2017-11-15 17:34:07 -0800 796) * to a new page.
>

I remember now we do not need to call invalidate range because
invalidate_range_end() does call invalidate_range so it is fine.
Comments should be better thought. So existing code is fine.

Cheers,
Jérôme