Re: [PATCH v2 3/5] mm/notifier: add migration invalidation type

[Ralph Campbell <rcampbell@xxxxxxxxxx>]

On 7/20/20 11:40 AM, Jason Gunthorpe wrote:
> On Mon, Jul 13, 2020 at 10:21:47AM -0700, Ralph Campbell wrote:
> > Currently migrate_vma_setup() calls mmu_notifier_invalidate_range_start()
> > which flushes all device private page mappings whether or not a page
> > is being migrated to/from device private memory. In order to not disrupt
> > device mappings that are not being migrated, shift the responsibility
> > for clearing device private mappings to the device driver and leave
> > CPU page table unmapping handled by migrate_vma_setup(). To support
> > this, the caller of migrate_vma_setup() should always set struct
> > migrate_vma::src_owner to a non NULL value that matches the device
> > private page->pgmap->owner. This value is then passed to the struct
> > mmu_notifier_range with a new event type which the driver's invalidation
> > function can use to avoid device MMU invalidations.
> >
> > Signed-off-by: Ralph Campbell <rcampbell@xxxxxxxxxx>
> >   include/linux/mmu_notifier.h | 7 +++++++
> >   mm/migrate.c                 | 8 +++++++-
> >   2 files changed, 14 insertions(+), 1 deletion(-)
> >
> > diff --git a/include/linux/mmu_notifier.h b/include/linux/mmu_notifier.h
> > index fc68f3570e19..1921fcf6be5b 100644
> > +++ b/include/linux/mmu_notifier.h
> > @@ -38,6 +38,10 @@ struct mmu_interval_notifier;
> >    *
> >    * @MMU_NOTIFY_RELEASE: used during mmu_interval_notifier invalidate to signal
> >    * that the mm refcount is zero and the range is no longer accessible.
> > + *
> > + * @MMU_NOTIFY_MIGRATE: used during migrate_vma_collect() invalidate to signal
> > + * a device driver to possibly ignore the invalidation if the
> > + * migrate_pgmap_owner field matches the driver's device private pgmap owner.
> >    */
> >   enum mmu_notifier_event {
> >   	MMU_NOTIFY_UNMAP = 0,
> > @@ -46,6 +50,7 @@ enum mmu_notifier_event {
> >   	MMU_NOTIFY_PROTECTION_PAGE,
> >   	MMU_NOTIFY_SOFT_DIRTY,
> >   	MMU_NOTIFY_RELEASE,
> > +	MMU_NOTIFY_MIGRATE,
> >   };
> >   
> >   #define MMU_NOTIFIER_RANGE_BLOCKABLE (1 << 0)
> > @@ -264,6 +269,7 @@ struct mmu_notifier_range {
> >   	unsigned long end;
> >   	unsigned flags;
> >   	enum mmu_notifier_event event;
> > +	void *migrate_pgmap_owner;
> >   };
> >   
> >   static inline int mm_has_notifiers(struct mm_struct *mm)
> > @@ -513,6 +519,7 @@ static inline void mmu_notifier_range_init(struct mmu_notifier_range *range,
> >   	range->start = start;
> >   	range->end = end;
> >   	range->flags = flags;
> > +	range->migrate_pgmap_owner = NULL;
> >   }
> >   
> >   #define ptep_clear_flush_young_notify(__vma, __address, __ptep)		\
> > diff --git a/mm/migrate.c b/mm/migrate.c
> > index 2bbc5c4c672e..9b3dcb81be5f 100644
> > +++ b/mm/migrate.c
> > @@ -2391,8 +2391,14 @@ static void migrate_vma_collect(struct migrate_vma *migrate)
> >   {
> >   	struct mmu_notifier_range range;
> >   
> > -	mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, NULL,
> > +	/*
> > +	 * Note that the src_owner is passed to the mmu notifier callback so
> > +	 * that the registered device driver can skip invalidating device
> > +	 * private page mappings that won't be migrated.
> > +	 */
> > +	mmu_notifier_range_init(&range, MMU_NOTIFY_MIGRATE, 0, migrate->vma,
> >   			migrate->vma->vm_mm, migrate->start, migrate->end);
>
> So the idea is that src_owner is always set to the pgmap owner when
> working with DEVICE_PRIVATE?
>
> But then the comment in the prior patch should be fixed:
>
> @@ -199,11 +204,12 @@  struct migrate_vma {
>   
>   	/*
>   	 * Set to the owner value also stored in page->pgmap->owner for
> +	 * migrating device private memory. The direction also needs to
> +	 * be set to MIGRATE_VMA_FROM_DEVICE_PRIVATE.
>
> To say the caller must always provide src_owner.
>
> And that field should probably be renamed at this point, as there is
> nothing "src" about it. It is just the pgmap_owner of the
> DEVICE_PRIVATE pages the TO/FROM DEVICE migration is working on.
>
> Jason

Good point. I'll send a v3 with your suggested changes.