linux-kernel - Re: [v5 06/15] mm/migrate_device: implement THP migration of zone device pages

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Message-ID: <49039b9d-4c42-480f-a219-daf0958be28e@nvidia.com>
Date: Fri, 12 Sep 2025 15:04:50 +1000
From: Balbir Singh <balbirs@...dia.com>
To: Mika Penttilä <mpenttil@...hat.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: [v5 06/15] mm/migrate_device: implement THP migration of zone
 device pages

On 9/11/25 21:52, Mika Penttilä wrote:
> sending again for the v5 thread..
> 
> On 9/8/25 03:04, 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 1f0ac122c3bf..41b4cc05a450 100644
>> --- a/include/linux/migrate.h
>> +++ b/include/linux/migrate.h
>> @@ -125,6 +125,7 @@ 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)
>> @@ -143,6 +144,7 @@ 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 f45ef182287d..1dfcf4799ea5 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 <linux/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
> 

Not really, if pfn is 0, we do a vm_insert_page (please see if (!page) line 1107) and
folio  handling in migrate_vma_finalize line 1284

Thanks,
Balbir