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 <a...@linux-foundation.org>
>> Cc: David Hildenbrand <da...@redhat.com>
>> Cc: Zi Yan <z...@nvidia.com>
>> Cc: Joshua Hahn <joshua.hah...@gmail.com>
>> Cc: Rakie Kim <rakie....@sk.com>
>> Cc: Byungchul Park <byungc...@sk.com>
>> Cc: Gregory Price <gou...@gourry.net>
>> Cc: Ying Huang <ying.hu...@linux.alibaba.com>
>> Cc: Alistair Popple <apop...@nvidia.com>
>> Cc: Oscar Salvador <osalva...@suse.de>
>> Cc: Lorenzo Stoakes <lorenzo.stoa...@oracle.com>
>> Cc: Baolin Wang <baolin.w...@linux.alibaba.com>
>> Cc: "Liam R. Howlett" <liam.howl...@oracle.com>
>> Cc: Nico Pache <npa...@redhat.com>
>> Cc: Ryan Roberts <ryan.robe...@arm.com>
>> Cc: Dev Jain <dev.j...@arm.com>
>> Cc: Barry Song <bao...@kernel.org>
>> Cc: Lyude Paul <ly...@redhat.com>
>> Cc: Danilo Krummrich <d...@kernel.org>
>> Cc: David Airlie <airl...@gmail.com>
>> Cc: Simona Vetter <sim...@ffwll.ch>
>> Cc: Ralph Campbell <rcampb...@nvidia.com>
>> Cc: Mika Penttilä <mpent...@redhat.com>
>> Cc: Matthew Brost <matthew.br...@intel.com>
>> Cc: Francois Dugast <francois.dug...@intel.com>
>>
>> Signed-off-by: Balbir Singh <balb...@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
Thanks,
Balbir
