Re: [PATCH 0/2] Optimize mmu_notifier->invalidate_range callback

[Jerome Glisse <jglisse@redhat.com>]

On Thu, Oct 19, 2017 at 01:43:19PM +1100, Balbir Singh wrote:
> On Mon, 16 Oct 2017 23:10:01 -0400
> jglisse@redhat.com wrote:
> 
> > From: Jérôme Glisse <jglisse@redhat.com>
> > 
> > (Andrew you already have v1 in your queue of patch 1, patch 2 is new,
> >  i think you can drop it patch 1 v1 for v2, v2 is bit more conservative
> >  and i fixed typos)
> > 
> > All this only affect user of invalidate_range callback (at this time
> > CAPI arch/powerpc/platforms/powernv/npu-dma.c, IOMMU ATS/PASID in
> > drivers/iommu/amd_iommu_v2.c|intel-svm.c)
> > 
> > This patchset remove useless double call to mmu_notifier->invalidate_range
> > callback wherever it is safe to do so. The first patch just remove useless
> > call
> 
> As in an extra call? Where does that come from?

Before this patch you had the following pattern:
  mmu_notifier_invalidate_range_start();
  take_page_table_lock()
  ...
  update_page_table()
  mmu_notifier_invalidate_range()
  ...
  drop_page_table_lock()
  mmu_notifier_invalidate_range_end();

It happens that mmu_notifier_invalidate_range_end() also make an
unconditional call to mmu_notifier_invalidate_range() so in the
above scenario you had 2 calls to mmu_notifier_invalidate_range()

Obviously one of the 2 call is useless. In some case you can drop
the first call (under the page table lock) this is what patch 1
does.

In other cases you can drop the second call that happen inside
mmu_notifier_invalidate_range_end() that is what patch 2 does.

Hence why i am referring to useless double call. I have added
more documentation to explain all this in the code and also under
Documentation/vm/mmu_notifier.txt


> 
> > and add documentation explaining why it is safe to do so. The second
> > patch go further by introducing mmu_notifier_invalidate_range_only_end()
> > which skip callback to invalidate_range this can be done when clearing a
> > pte, pmd or pud with notification which call invalidate_range right after
> > clearing under the page table lock.
> >
> 
> Balbir Singh.
>