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

["Mika Penttilä" <mpenttil@redhat.com>]

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@linux-foundation.org>
>>>> Cc: David Hildenbrand <david@redhat.com>
>>>> Cc: Zi Yan <ziy@nvidia.com>
>>>> Cc: Joshua Hahn <joshua.hahnjy@gmail.com>
>>>> Cc: Rakie Kim <rakie.kim@sk.com>
>>>> Cc: Byungchul Park <byungchul@sk.com>
>>>> Cc: Gregory Price <gourry@gourry.net>
>>>> Cc: Ying Huang <ying.huang@linux.alibaba.com>
>>>> Cc: Alistair Popple <apopple@nvidia.com>
>>>> Cc: Oscar Salvador <osalvador@suse.de>
>>>> Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
>>>> Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
>>>> Cc: "Liam R. Howlett" <Liam.Howlett@oracle.com>
>>>> Cc: Nico Pache <npache@redhat.com>
>>>> Cc: Ryan Roberts <ryan.roberts@arm.com>
>>>> Cc: Dev Jain <dev.jain@arm.com>
>>>> Cc: Barry Song <baohua@kernel.org>
>>>> 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: Ralph Campbell <rcampbell@nvidia.com>
>>>> Cc: Mika Penttilä <mpenttil@redhat.com>
>>>> Cc: Matthew Brost <matthew.brost@intel.com>
>>>> Cc: Francois Dugast <francois.dugast@intel.com>
>>>>
>>>> Signed-off-by: Balbir Singh <balbirs@nvidia.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
> 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.

>> Thanks,
>> Balbir
>>
> --Mika
>