Re: [v1 resend 02/12] mm/migrate_device: flags for selecting device private THP pages

[Matthew Brost <matthew.brost@intel.com>]

On Fri, Jul 04, 2025 at 09:35:01AM +1000, Balbir Singh wrote:
> Add flags to mark zone device migration pages.
> 
> MIGRATE_VMA_SELECT_COMPOUND will be used to select THP pages during
> migrate_vma_setup() and MIGRATE_PFN_COMPOUND will make migrating
> device pages as compound pages during device pfn migration.
> 
> Cc: Karol Herbst <kherbst@redhat.com>
> Cc: Lyude Paul <lyude@redhat.com>
> Cc: Danilo Krummrich <dakr@kernel.org>
> Cc: David Airlie <airlied@gmail.com>
> Cc: Simona Vetter <simona@ffwll.ch>
> Cc: "Jérôme Glisse" <jglisse@redhat.com>
> Cc: Shuah Khan <shuah@kernel.org>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: Barry Song <baohua@kernel.org>
> Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
> Cc: Ryan Roberts <ryan.roberts@arm.com>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: Peter Xu <peterx@redhat.com>
> Cc: Zi Yan <ziy@nvidia.com>
> Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
> Cc: Jane Chu <jane.chu@oracle.com>
> Cc: Alistair Popple <apopple@nvidia.com>
> Cc: Donet Tom <donettom@linux.ibm.com>
> 
> Signed-off-by: Balbir Singh <balbirs@nvidia.com>
> ---
>  include/linux/migrate.h | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/include/linux/migrate.h b/include/linux/migrate.h
> index aaa2114498d6..1661e2d5479a 100644
> --- a/include/linux/migrate.h
> +++ b/include/linux/migrate.h
> @@ -167,6 +167,7 @@ static inline int migrate_misplaced_folio(struct folio *folio, int node)
>  #define MIGRATE_PFN_VALID	(1UL << 0)
>  #define MIGRATE_PFN_MIGRATE	(1UL << 1)
>  #define MIGRATE_PFN_WRITE	(1UL << 3)
> +#define MIGRATE_PFN_COMPOUND	(1UL << 4)

Can some documentation be added around the usage of MIGRATE_PFN_COMPOUND?

In particular, how the field is used in relation to the migrate_vma_* functions?

For example, when MIGRATE_PFN_COMPOUND is set in a returned mpfn, the caller
should check the order of the folio associated with that mpfn, and then expect
the next 1 << order entries in the source array to be unpopulated.

Likewise, when a caller populates an mpfn with MIGRATE_PFN_COMPOUND, the next 1
<< order entries should also be unpopulated.

This behavior wasn’t immediately obvious, so I think it would be helpful to
document it to avoid requiring readers to reverse-engineer the code.

>  #define MIGRATE_PFN_SHIFT	6
>  
>  static inline struct page *migrate_pfn_to_page(unsigned long mpfn)
> @@ -185,6 +186,7 @@ enum migrate_vma_direction {

Maybe out of scope, but migrate_vma_direction is not a great name. While
we are here, it could be cleaned up.

>  	MIGRATE_VMA_SELECT_SYSTEM = 1 << 0,
>  	MIGRATE_VMA_SELECT_DEVICE_PRIVATE = 1 << 1,
>  	MIGRATE_VMA_SELECT_DEVICE_COHERENT = 1 << 2,
> +	MIGRATE_VMA_SELECT_COMPOUND = 1 << 3,

Same here.

For example, what happens if MIGRATE_VMA_SELECT_COMPOUND is select vs unselected
when higher order folio is found.

Matt

>  };
>  
>  struct migrate_vma {
> -- 
> 2.49.0
>