linux-kernel - Re: [v4 06/15] mm/migrate_device: implement THP migration of zone device pages

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Message-ID: <9047198d-7b35-435b-a933-ff7b1357919b@redhat.com>
Date: Thu, 11 Sep 2025 14:11:40 +0300
From: Mika Penttilä <mpenttil@...hat.com>
To: Balbir Singh <balbirs@...dia.com>, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org
Cc: damon@...ts.linux.dev, dri-devel@...ts.freedesktop.org,
 Andrew Morton <akpm@...ux-foundation.org>,
 David Hildenbrand <david@...hat.com>, Zi Yan <ziy@...dia.com>,
 Joshua Hahn <joshua.hahnjy@...il.com>, Rakie Kim <rakie.kim@...com>,
 Byungchul Park <byungchul@...com>, Gregory Price <gourry@...rry.net>,
 Ying Huang <ying.huang@...ux.alibaba.com>,
 Alistair Popple <apopple@...dia.com>, Oscar Salvador <osalvador@...e.de>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Baolin Wang <baolin.wang@...ux.alibaba.com>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>, Nico Pache <npache@...hat.com>,
 Ryan Roberts <ryan.roberts@....com>, Dev Jain <dev.jain@....com>,
 Barry Song <baohua@...nel.org>, Lyude Paul <lyude@...hat.com>,
 Danilo Krummrich <dakr@...nel.org>, David Airlie <airlied@...il.com>,
 Simona Vetter <simona@...ll.ch>, Ralph Campbell <rcampbell@...dia.com>,
 Matthew Brost <matthew.brost@...el.com>,
 Francois Dugast <francois.dugast@...el.com>
Subject: Re: [v4 06/15] mm/migrate_device: implement THP migration of zone
 device pages

Hi,

On 9/3/25 04:18, Balbir Singh wrote:

> 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.
>
> migrate_device code paths go through the collect, setup
> and finalize phases of migration.
>
> The entries in src and dst arrays passed to these functions still
> remain at a PAGE_SIZE granularity. When a compound page is passed,
> the first entry has the PFN along with MIGRATE_PFN_COMPOUND
> and other flags set (MIGRATE_PFN_MIGRATE, MIGRATE_PFN_VALID), the
> remaining entries (HPAGE_PMD_NR - 1) are filled with 0's. This
> representation allows for the compound page to be split into smaller
> page sizes.
>
> migrate_vma_collect_hole(), migrate_vma_collect_pmd() are now THP
> page aware. Two new helper functions migrate_vma_collect_huge_pmd()
> and migrate_vma_insert_huge_pmd_page() have been added.
>
> migrate_vma_collect_huge_pmd() can collect THP pages, but if for
> some reason this fails, there is fallback support to split the folio
> and migrate it.
>
> migrate_vma_insert_huge_pmd_page() closely follows the logic of
> migrate_vma_insert_page()
>
> Support for splitting pages as needed for migration will follow in
> later patches in this series.
>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: David Hildenbrand <david@...hat.com>
> Cc: Zi Yan <ziy@...dia.com>
> Cc: Joshua Hahn <joshua.hahnjy@...il.com>
> Cc: Rakie Kim <rakie.kim@...com>
> Cc: Byungchul Park <byungchul@...com>
> Cc: Gregory Price <gourry@...rry.net>
> Cc: Ying Huang <ying.huang@...ux.alibaba.com>
> Cc: Alistair Popple <apopple@...dia.com>
> Cc: Oscar Salvador <osalvador@...e.de>
> Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> Cc: Baolin Wang <baolin.wang@...ux.alibaba.com>
> Cc: "Liam R. Howlett" <Liam.Howlett@...cle.com>
> Cc: Nico Pache <npache@...hat.com>
> Cc: Ryan Roberts <ryan.roberts@....com>
> Cc: Dev Jain <dev.jain@....com>
> Cc: Barry Song <baohua@...nel.org>
> 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: Ralph Campbell <rcampbell@...dia.com>
> Cc: Mika Penttilä <mpenttil@...hat.com>
> Cc: Matthew Brost <matthew.brost@...el.com>
> Cc: Francois Dugast <francois.dugast@...el.com>
>
> Signed-off-by: Balbir Singh <balbirs@...dia.com>
> ---
>  include/linux/migrate.h |   2 +
>  mm/migrate_device.c     | 456 ++++++++++++++++++++++++++++++++++------
>  2 files changed, 395 insertions(+), 63 deletions(-)
>
> diff --git a/include/linux/migrate.h b/include/linux/migrate.h
> index 9009e27b5f44..40e1c792eb54 100644
> --- a/include/linux/migrate.h
> +++ b/include/linux/migrate.h
> @@ -134,6 +134,7 @@ 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)
>  #define MIGRATE_PFN_SHIFT	6
>  
>  static inline struct page *migrate_pfn_to_page(unsigned long mpfn)
> @@ -152,6 +153,7 @@ enum migrate_vma_direction {
>  	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,
>  };
>  
>  struct migrate_vma {
> diff --git a/mm/migrate_device.c b/mm/migrate_device.c
> index e58c3f9d01c8..aba0cd7856da 100644
> --- a/mm/migrate_device.c
> +++ b/mm/migrate_device.c
> @@ -14,6 +14,7 @@
>  #include <linux/pagewalk.h>
>  #include <linux/rmap.h>
>  #include <linux/swapops.h>
> +#include <asm/pgalloc.h>
>  #include <asm/tlbflush.h>
>  #include "internal.h"
>  
> @@ -44,6 +45,23 @@ static int migrate_vma_collect_hole(unsigned long start,
>  	if (!vma_is_anonymous(walk->vma))
>  		return migrate_vma_collect_skip(start, end, walk);
>  
> +	if (thp_migration_supported() &&
> +		(migrate->flags & MIGRATE_VMA_SELECT_COMPOUND) &&
> +		(IS_ALIGNED(start, HPAGE_PMD_SIZE) &&
> +		 IS_ALIGNED(end, HPAGE_PMD_SIZE))) {
> +		migrate->src[migrate->npages] = MIGRATE_PFN_MIGRATE |
> +						MIGRATE_PFN_COMPOUND;
> +		migrate->dst[migrate->npages] = 0;
> +		migrate->npages++;
> +		migrate->cpages++;
> +
> +		/*
> +		 * Collect the remaining entries as holes, in case we
> +		 * need to split later
> +		 */
> +		return migrate_vma_collect_skip(start + PAGE_SIZE, end, walk);
> +	}
> +

seems you have to split_huge_pmd() for the huge zero page here in case of !thp_migration_supported() afaics

>  	for (addr = start; addr < end; addr += PAGE_SIZE) {
>  		migrate->src[migrate->npages] = MIGRATE_PFN_MIGRATE;
>  		migrate->dst[migrate->npages] = 0;
> @@ -102,57 +120,150 @@ static int migrate_vma_split_folio(struct folio *folio,
>  	return 0;
>  }

--Mika