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

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Message-ID: <aHm8OJPDLd3muPC+@lstrano-desk.jf.intel.com>
Date: Thu, 17 Jul 2025 20:15:04 -0700
From: Matthew Brost <matthew.brost@...el.com>
To: Balbir Singh <balbirs@...dia.com>
CC: <linux-mm@...ck.org>, <akpm@...ux-foundation.org>,
	<linux-kernel@...r.kernel.org>, Karol Herbst <kherbst@...hat.com>, Lyude Paul
	<lyude@...hat.com>, Danilo Krummrich <dakr@...nel.org>, David Airlie
	<airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
	Jérôme Glisse <jglisse@...hat.com>, Shuah Khan
	<shuah@...nel.org>, David Hildenbrand <david@...hat.com>, Barry Song
	<baohua@...nel.org>, Baolin Wang <baolin.wang@...ux.alibaba.com>, "Ryan
 Roberts" <ryan.roberts@....com>, Matthew Wilcox <willy@...radead.org>, "Peter
 Xu" <peterx@...hat.com>, Zi Yan <ziy@...dia.com>, Kefeng Wang
	<wangkefeng.wang@...wei.com>, Jane Chu <jane.chu@...cle.com>, Alistair Popple
	<apopple@...dia.com>, Donet Tom <donettom@...ux.ibm.com>
Subject: Re: [v1 resend 02/12] mm/migrate_device: flags for selecting device
 private THP pages

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@...hat.com>
> Cc: Lyude Paul <lyude@...hat.com>
> Cc: Danilo Krummrich <dakr@...nel.org>
> Cc: David Airlie <airlied@...il.com>
> Cc: Simona Vetter <simona@...ll.ch>
> Cc: "Jérôme Glisse" <jglisse@...hat.com>
> Cc: Shuah Khan <shuah@...nel.org>
> Cc: David Hildenbrand <david@...hat.com>
> Cc: Barry Song <baohua@...nel.org>
> Cc: Baolin Wang <baolin.wang@...ux.alibaba.com>
> Cc: Ryan Roberts <ryan.roberts@....com>
> Cc: Matthew Wilcox <willy@...radead.org>
> Cc: Peter Xu <peterx@...hat.com>
> Cc: Zi Yan <ziy@...dia.com>
> Cc: Kefeng Wang <wangkefeng.wang@...wei.com>
> Cc: Jane Chu <jane.chu@...cle.com>
> Cc: Alistair Popple <apopple@...dia.com>
> Cc: Donet Tom <donettom@...ux.ibm.com>
> 
> Signed-off-by: Balbir Singh <balbirs@...dia.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
>