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

[Balbir Singh]

On 9/12/25 15:38, Mika Penttilä wrote:
> 
> On 9/12/25 08:28, Mika Penttilä wrote:
> 
>> On 9/12/25 08:04, Balbir Singh wrote:
>>
>>> 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@xxxxxxxxxxxxxxxxxxxx>
>>>>> Cc: David Hildenbrand <david@xxxxxxxxxx>
>>>>> Cc: Zi Yan <ziy@xxxxxxxxxx>
>>>>> Cc: Joshua Hahn <joshua.hahnjy@xxxxxxxxx>
>>>>> Cc: Rakie Kim <rakie.kim@xxxxxx>
>>>>> Cc: Byungchul Park <byungchul@xxxxxx>
>>>>> Cc: Gregory Price <gourry@xxxxxxxxxx>
>>>>> Cc: Ying Huang <ying.huang@xxxxxxxxxxxxxxxxx>
>>>>> Cc: Alistair Popple <apopple@xxxxxxxxxx>
>>>>> Cc: Oscar Salvador <osalvador@xxxxxxx>
>>>>> Cc: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
>>>>> Cc: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
>>>>> Cc: "Liam R. Howlett" <Liam.Howlett@xxxxxxxxxx>
>>>>> Cc: Nico Pache <npache@xxxxxxxxxx>
>>>>> Cc: Ryan Roberts <ryan.roberts@xxxxxxx>
>>>>> Cc: Dev Jain <dev.jain@xxxxxxx>
>>>>> Cc: Barry Song <baohua@xxxxxxxxxx>
>>>>> Cc: Lyude Paul <lyude@xxxxxxxxxx>
>>>>> Cc: Danilo Krummrich <dakr@xxxxxxxxxx>
>>>>> Cc: David Airlie <airlied@xxxxxxxxx>
>>>>> Cc: Simona Vetter <simona@xxxxxxxx>
>>>>> Cc: Ralph Campbell <rcampbell@xxxxxxxxxx>
>>>>> Cc: Mika Penttilä <mpenttil@xxxxxxxxxx>
>>>>> Cc: Matthew Brost <matthew.brost@xxxxxxxxx>
>>>>> Cc: Francois Dugast <francois.dugast@xxxxxxxxx>
>>>>>
>>>>> Signed-off-by: Balbir Singh <balbirs@xxxxxxxxxx>
>>>>> ---
>>>>>  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
>> Ok actually seems it is handled by migrate_vma_insert_page() which does
>>
>>         if (!pmd_none(*pmdp)) {
>>                 if (pmd_trans_huge(*pmdp)) {
>>                         if (!is_huge_zero_pmd(*pmdp))
>>                                 goto abort;
>>                         folio_get(pmd_folio(*pmdp));
>>                         split_huge_pmd(vma, pmdp, addr);   <----- here
>>                 } else if (pmd_leaf(*pmdp))
>>                         goto abort;
>>         }
>>
> While at it, think the folio_get(pmd_folio(*pmdp)); is wrong for here,
> we split the pmd for huge zero page.
> 

Ack, will do

Thanks for the review
Balbir