[PATCH v1 1/8] mm/migrate_device: Add migrate PFN flag to track device private pages
Alistair Popple
apopple at nvidia.com
Mon Jan 5 11:36:32 AEDT 2026
On 2026-01-01 at 04:03 +1100, "Kuehling, Felix" <felix.kuehling at amd.com> wrote...
> On 2025-12-30 23:31, Jordan Niethe wrote:
> > A future change will remove device private pages from the physical
> > address space. This will mean that device private pages no longer have
> > normal PFN and must be handled separately.
> >
> > Prepare for this by adding a MIGRATE_PFN_DEVICE flag to indicate
> > that a migrate pfn contains a PFN for a device private page.
>
> Thanks for doing this. Some comments inline regarding DEVICE_COHERENT pages.
> I suspect this will have ripple effects on the rest of the patch series, at
> least in patch 8, but I haven't looked at that in detail yet.
Thanks for catching the DEVICE_COHERENT aspects, they need to remain as real
PFNs. I think the flag should be renamed to MIGRATE_PFN_DEVICE_PRIVATE to make
this explicit.
> >
> > Signed-off-by: Jordan Niethe <jniethe at nvidia.com>
> > Signed-off-by: Alistair Popple <apopple at nvidia.com>
> >
> > ---
> > v1:
> > - Update for HMM huge page support
> > - Update existing drivers to use MIGRATE_PFN_DEVICE
> > ---
> > arch/powerpc/kvm/book3s_hv_uvmem.c | 2 +-
> > drivers/gpu/drm/amd/amdkfd/kfd_migrate.c | 3 ++-
> > drivers/gpu/drm/drm_pagemap.c | 2 +-
> > drivers/gpu/drm/nouveau/nouveau_dmem.c | 2 +-
> > include/linux/migrate.h | 1 +
> > lib/test_hmm.c | 4 ++--
> > mm/migrate_device.c | 11 ++++++++---
> > 7 files changed, 16 insertions(+), 9 deletions(-)
> >
> > diff --git a/arch/powerpc/kvm/book3s_hv_uvmem.c b/arch/powerpc/kvm/book3s_hv_uvmem.c
> > index e5000bef90f2..dac5d6454920 100644
> > --- a/arch/powerpc/kvm/book3s_hv_uvmem.c
> > +++ b/arch/powerpc/kvm/book3s_hv_uvmem.c
> > @@ -784,7 +784,7 @@ static int kvmppc_svm_page_in(struct vm_area_struct *vma,
> > }
> > }
> > - *mig.dst = migrate_pfn(page_to_pfn(dpage));
> > + *mig.dst = migrate_pfn(page_to_pfn(dpage)) | MIGRATE_PFN_DEVICE;
> > migrate_vma_pages(&mig);
> > out_finalize:
> > migrate_vma_finalize(&mig);
> > diff --git a/drivers/gpu/drm/amd/amdkfd/kfd_migrate.c b/drivers/gpu/drm/amd/amdkfd/kfd_migrate.c
> > index af53e796ea1b..0257c6e7f680 100644
> > --- a/drivers/gpu/drm/amd/amdkfd/kfd_migrate.c
> > +++ b/drivers/gpu/drm/amd/amdkfd/kfd_migrate.c
> > @@ -303,7 +303,8 @@ svm_migrate_copy_to_vram(struct kfd_node *node, struct svm_range *prange,
> > dst[i] = cursor.start + (j << PAGE_SHIFT);
> > migrate->dst[i] = svm_migrate_addr_to_pfn(adev, dst[i]);
> > svm_migrate_get_vram_page(prange, migrate->dst[i]);
> > - migrate->dst[i] = migrate_pfn(migrate->dst[i]);
> > + migrate->dst[i] = migrate_pfn(migrate->dst[i]) |
> > + MIGRATE_PFN_DEVICE;
>
> On some of our GPUs we use DEVICE_COHERENT pages. These are pages that are
> coherently accessible by the CPU and peer devices in the system physical
> address space. Therefore, this needs to be conditional. Maybe add something
> like adev->kfd.migrate_pfn_flag that gets initialized conditionally in
> kgd2kfd_init_zone_device. Then add ... | adev->kfd.migrate_pfn_flag here.
I don't think you need a new flag, you could just make it conditional on
adev->kfd.pgmap.type == MEMORY_DEVICE_PRIVATE.
Note the need for this conditional is specific to the AMD driver and HMM tests -
AFAIK there are no other uses of DEVICE_COHERENT in the kernel.
- Alistair
> > mpages++;
> > }
> > spage = migrate_pfn_to_page(migrate->src[i]);
> > diff --git a/drivers/gpu/drm/drm_pagemap.c b/drivers/gpu/drm/drm_pagemap.c
> > index 37d7cfbbb3e8..0c756d73419f 100644
> > --- a/drivers/gpu/drm/drm_pagemap.c
> > +++ b/drivers/gpu/drm/drm_pagemap.c
> > @@ -404,7 +404,7 @@ int drm_pagemap_migrate_to_devmem(struct drm_pagemap_devmem *devmem_allocation,
> > struct page *page = pfn_to_page(migrate.dst[i]);
> > pages[i] = page;
> > - migrate.dst[i] = migrate_pfn(migrate.dst[i]);
> > + migrate.dst[i] = migrate_pfn(migrate.dst[i]) | MIGRATE_PFN_DEVICE;
> > drm_pagemap_get_devmem_page(page, zdd);
> > }
> > diff --git a/drivers/gpu/drm/nouveau/nouveau_dmem.c b/drivers/gpu/drm/nouveau/nouveau_dmem.c
> > index 58071652679d..2bd80c6f5bcd 100644
> > --- a/drivers/gpu/drm/nouveau/nouveau_dmem.c
> > +++ b/drivers/gpu/drm/nouveau/nouveau_dmem.c
> > @@ -766,7 +766,7 @@ static unsigned long nouveau_dmem_migrate_copy_one(struct nouveau_drm *drm,
> > ((paddr >> PAGE_SHIFT) << NVIF_VMM_PFNMAP_V0_ADDR_SHIFT);
> > if (src & MIGRATE_PFN_WRITE)
> > *pfn |= NVIF_VMM_PFNMAP_V0_W;
> > - mpfn = migrate_pfn(page_to_pfn(dpage));
> > + mpfn = migrate_pfn(page_to_pfn(dpage)) | MIGRATE_PFN_DEVICE;
> > if (folio_order(page_folio(dpage)))
> > mpfn |= MIGRATE_PFN_COMPOUND;
> > return mpfn;
> > diff --git a/include/linux/migrate.h b/include/linux/migrate.h
> > index 26ca00c325d9..52f65cd5c932 100644
> > --- a/include/linux/migrate.h
> > +++ b/include/linux/migrate.h
> > @@ -126,6 +126,7 @@ static inline int migrate_misplaced_folio(struct folio *folio, int node)
> > #define MIGRATE_PFN_MIGRATE (1UL << 1)
> > #define MIGRATE_PFN_WRITE (1UL << 3)
> > #define MIGRATE_PFN_COMPOUND (1UL << 4)
> > +#define MIGRATE_PFN_DEVICE (1UL << 5)
> > #define MIGRATE_PFN_SHIFT 6
> > static inline struct page *migrate_pfn_to_page(unsigned long mpfn)
> > diff --git a/lib/test_hmm.c b/lib/test_hmm.c
> > index 8af169d3873a..19681904a666 100644
> > --- a/lib/test_hmm.c
> > +++ b/lib/test_hmm.c
> > @@ -727,7 +727,7 @@ static void dmirror_migrate_alloc_and_copy(struct migrate_vma *args,
> > rpage = BACKING_PAGE(dpage);
> > rpage->zone_device_data = dmirror;
> > - *dst = migrate_pfn(page_to_pfn(dpage)) | write;
> > + *dst = migrate_pfn(page_to_pfn(dpage)) | MIGRATE_PFN_DEVICE | write;
>
> This needs to be conditional on dmirror->mdevice->zone_device_type.
>
>
> > src_page = pfn_to_page(spfn + i);
> > if (spage)
> > @@ -754,7 +754,7 @@ static void dmirror_migrate_alloc_and_copy(struct migrate_vma *args,
> > pr_debug("migrating from sys to dev pfn src: 0x%lx pfn dst: 0x%lx\n",
> > page_to_pfn(spage), page_to_pfn(dpage));
> > - *dst = migrate_pfn(page_to_pfn(dpage)) | write;
> > + *dst = migrate_pfn(page_to_pfn(dpage)) | MIGRATE_PFN_DEVICE | write;
>
> Same here.
>
>
> > if (is_large) {
> > int i;
> > diff --git a/mm/migrate_device.c b/mm/migrate_device.c
> > index 23379663b1e1..5d108ddf1a97 100644
> > --- a/mm/migrate_device.c
> > +++ b/mm/migrate_device.c
> > @@ -199,6 +199,7 @@ static int migrate_vma_collect_huge_pmd(pmd_t *pmdp, unsigned long start,
> > (migrate->flags & MIGRATE_VMA_SELECT_COMPOUND) &&
> > (IS_ALIGNED(start, HPAGE_PMD_SIZE) &&
> > IS_ALIGNED(end, HPAGE_PMD_SIZE))) {
> > + unsigned long device_private = 0;
> > struct page_vma_mapped_walk pvmw = {
> > .ptl = ptl,
> > @@ -208,10 +209,13 @@ static int migrate_vma_collect_huge_pmd(pmd_t *pmdp, unsigned long start,
> > };
> > unsigned long pfn = page_to_pfn(folio_page(folio, 0));
> > + if (folio_is_device_private(folio))
> > + device_private = MIGRATE_PFN_DEVICE;
> > migrate->src[migrate->npages] = migrate_pfn(pfn) | write
> > | MIGRATE_PFN_MIGRATE
> > - | MIGRATE_PFN_COMPOUND;
> > + | MIGRATE_PFN_COMPOUND
> > + | device_private;
> > migrate->dst[migrate->npages++] = 0;
> > migrate->cpages++;
> > ret = set_pmd_migration_entry(&pvmw, folio_page(folio, 0));
> > @@ -329,7 +333,8 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
> > }
> > mpfn = migrate_pfn(page_to_pfn(page)) |
> > - MIGRATE_PFN_MIGRATE;
> > + MIGRATE_PFN_MIGRATE |
> > + MIGRATE_PFN_DEVICE;
>
> I think this also needs to be conditional to distinguish DEVICE_COHERENT
> pages.
>
>
> > if (softleaf_is_device_private_write(entry))
> > mpfn |= MIGRATE_PFN_WRITE;
> > } else {
> > @@ -1368,7 +1373,7 @@ static unsigned long migrate_device_pfn_lock(unsigned long pfn)
> > return 0;
> > }
> > - return migrate_pfn(pfn) | MIGRATE_PFN_MIGRATE;
> > + return migrate_pfn(pfn) | MIGRATE_PFN_MIGRATE | MIGRATE_PFN_DEVICE;
>
> Same here.
>
> Regards,
> Felix
>
>
> > }
> > /**
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