Re: [RFC PATCH 04/28] mm/migrate: Add migrate_device_vma_range

[Daniel Vetter <daniel.vetter@ffwll.ch>]

On Tue, Aug 27, 2024 at 07:48:37PM -0700, Matthew Brost wrote:
> Add migrate_device_vma_range which prepares an array of pre-populated
> device pages for migration and issues a MMU invalidation.
> 
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Signed-off-by: Matthew Brost <matthew.brost@intel.com>
> ---
>  include/linux/migrate.h |  3 +++
>  mm/migrate_device.c     | 53 +++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 56 insertions(+)
> 
> diff --git a/include/linux/migrate.h b/include/linux/migrate.h
> index 644be30b69c8..e8cce05bf9c2 100644
> --- a/include/linux/migrate.h
> +++ b/include/linux/migrate.h
> @@ -226,6 +226,9 @@ void migrate_vma_pages(struct migrate_vma *migrate);
>  void migrate_vma_finalize(struct migrate_vma *migrate);
>  int migrate_device_range(unsigned long *src_pfns, unsigned long start,
>  			unsigned long npages);
> +int migrate_device_vma_range(struct mm_struct *mm, void *pgmap_owner,
> +			     unsigned long *src_pfns, unsigned long npages,
> +			     unsigned long start);
>  void migrate_device_pages(unsigned long *src_pfns, unsigned long *dst_pfns,
>  			unsigned long npages);
>  void migrate_device_finalize(unsigned long *src_pfns,
> diff --git a/mm/migrate_device.c b/mm/migrate_device.c
> index 6d66dc1c6ffa..e25f12a132e8 100644
> --- a/mm/migrate_device.c
> +++ b/mm/migrate_device.c
> @@ -920,6 +920,59 @@ int migrate_device_range(unsigned long *src_pfns, unsigned long start,
>  }
>  EXPORT_SYMBOL(migrate_device_range);
>  
> +/**
> + * migrate_device_vma_range() - migrate device private pfns to normal memory and
> + * trigger MMU invalidation.
> + * @mm: struct mm of device pages.
> + * @src_pfns: pre-popluated array of source device private pfns to migrate.
> + * @pgmap_owner: page group map owner of device pages.
> + * @npages: number of pages to migrate.
> + * @start: VMA start of device pages.
> + *
> + * Similar to migrate_device_range() but supports non-contiguous pre-popluated
> + * array of device pages to migrate. Also triggers MMU invalidation. Useful in
> + * device memory eviction paths where lock is held protecting the device pages
> + * but where the mmap lock cannot be taken to due to a locking inversion (e.g.
> + * DRM drivers). Since the mmap lock is not required to be held, the MMU
> + * invalidation can race with with VMA start being repurposed, worst case this
> + * would result in an unecessary invalidation.
> + */
> +int migrate_device_vma_range(struct mm_struct *mm, void *pgmap_owner,
> +			     unsigned long *src_pfns, unsigned long npages,
> +			     unsigned long start)
> +{
> +	struct mmu_notifier_range range;
> +	unsigned long i;
> +
> +	mmu_notifier_range_init_owner(&range, MMU_NOTIFY_MIGRATE, 0,
> +				      mm, start, start + npages * PAGE_SIZE,
> +				      pgmap_owner);
> +	mmu_notifier_invalidate_range_start(&range);

This isn't needed, try_to_migrate called from migrate_device_unmap already
has a notifier, if there's actually any ptes to clear. If you need this
one you've missed a pte clear notification somewhere, or there's some
other bad bug somewhere.
-Sima

> +
> +	for (i = 0; i < npages; i++) {
> +		struct page *page = pfn_to_page(src_pfns[i]);
> +
> +		if (!get_page_unless_zero(page)) {
> +			src_pfns[i] = 0;
> +			continue;
> +		}
> +
> +		if (!trylock_page(page)) {
> +			src_pfns[i] = 0;
> +			put_page(page);
> +			continue;
> +		}
> +
> +		src_pfns[i] = migrate_pfn(src_pfns[i]) | MIGRATE_PFN_MIGRATE;
> +	}
> +
> +	migrate_device_unmap(src_pfns, npages, NULL);
> +	mmu_notifier_invalidate_range_end(&range);
> +
> +	return 0;
> +}
> +EXPORT_SYMBOL(migrate_device_vma_range);
> +
>  /*
>   * Migrate a device coherent page back to normal memory. The caller should have
>   * a reference on page which will be copied to the new page if migration is
> -- 
> 2.34.1
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch